POPULAR  GUIDE 


LIBRARY 

OF  THK 

UNIVERSITY  OF  CALIFORNIA. 


Class 


A     POPULAR     GUIDE 


TO    THE 


HEAVENS. 


POPULAR    GUIDE 


TO 


THE     H  EAVENS 


A    SERIES    OF   EIGHTY-THREE    PLATES, 


WITH  EXPLANATORY  TEXT  &  INDEX. 


BY 

SIR  ROBERT  STAWELL  BALL,  LL.D.,  F.R.S., 

»\ 

LOWNDEAN    PROFESSOR   OF   ASTRONOMY  AND   GEOMETRY  IN  THE    UNIVERSITY  OF  CAMBRIDGE. 


OF  THE 

DIVERSITY 


D.   VAN   NOSTRAND   COMPANY, 

23     MURRAY     AND     27     WARREN     STREETS, 
NEW     YORK. 


GENERAL 


PREFACE. 


THE  object  of  the  present  work  is  to  provide  a  popular  guide  to  the  study  of  the  sky 
by  furnishing  a  summary  of  our  present  knowledge  of  the  Solar  system,  a  guide  to 
the  positions  of  the  planets  for  the  first  half  of  the  present  century,  a  series  of  star 
maps,  some  examples  of  the  finest  achievements  in  the  art  of  drawing  and  photographing 
celestial  objects,  and  a  list  of  interesting  objects  which  may  be  observed  with  small 
telescopes. 

In  the  text  will  be  found  a  descriptive  account  of  the  plates,  and  of  the  methods 
of  using  the  maps  and  tables.  It  is,  however,  desirable  to  draw  attention  in  this  place 
to  certain  characteristics  of  the  work,  and  to  make  my  acknowledgment  to  the 
friends  who  have  kindly  assisted  me. 

In  1892  I  edited  an  atlas  of  the  celestial  bodies  which  has  long  been  out  of  print. 
Though  this  atlas  bore  my  name  yet  as  explained  in  the  preface  it  was  largely  due 
to  my  friend  Dr.  Rarnbaut.  The  question  of  a  new  issue  of  the  work  having  arisen, 
it  was  deemed  better  to  recast  the  book  completely,  and  the  present  volume  is  the 
result. 

The  star  maps  carefully  drawn  by  Dr.  Rarnbaut  having  been  corrected  for  the 
changes  obviously  required  by  the  lapse  of  12  years  have  been  retained,  so  have  also 
the  maps  of  the  Moon,  drawn  by  the  late  Mr.  Elger. 

But  the  advance  of  Astronomical  portraiture  has  rendered  it  necessary  to 
supersede  most  of  the  remaining  plates  by  new  material.  This  has  involved  so  many 
changes  in  the  text  that  the  book  is  substantially  a  new  one,  and  is  now  arranged  in 
such  a  manner  as  may,  I  hope,  entitle  the  book  to  be  called  a  Popular  Guide  to  the 
Heavens. 

The  map  of  Mars  is  reduced  from  the  large  map  made  by  Mr.  A.  E.  Douglass, 
published  in  Volume  II.  of  the  Annals  of  the  Lowell  Observatory. 

The  drawings  of  Jupiter  on  plate  9  have  been  copied  from  Dr.  O.  Lohse's  obser- 
vations, in  the  third  volume  of  the  Astrophysiklische  Observatoriuin  at  Potsdam.  The 
drawings  of  Jupiter's  satellite  are  from  a  paper  by  Professor  Barnard,  published  in 
the  Monthly  Notices  of  the  Royal  Astronomical  Society.  The  drawing  of  Saturn,  plate 
10,  by  Professor  Barnard,  is  reproduced  from  a  drawing  also  published  in  the  Monthly 
Notices. 

179892 


vi.  PEEFACE. 

The  photograph  of  the  great  sunspot  of  1898  September  is  reproduced  from  a 
photograph  taken  at  the  Royal  Observatory,  Greenwich,  for  which  I  am  indebted  to 
the  kindness  of  the  Astronomer  Royal. 

The  drawings  of  the  Solar  Prominences  on  plate  12  are  from  a  paper  by  Herr 
Fenyi,  in  the  Aslrophysical  Journal ;  the  picture  of  the  Solar  Prominence  photographed 
by  Professor  Barnard  is  from  the  report  of  the  Yerkes  Observatory  Eclipse  Expedition 
to  Wadesboro',  U.S.A.,  published  in  the  same  journal. 

The  map  of  the  paths  of  Solar  Eclipses,  1901-1950,  has  been  prepared  from  the 
series  of  maps  in  Dr.  Oppolzer's  great  work,  Canon  der  Finsternisse. 

The  photographs  of  typical  Solar  Coronae  on  plate  16  are  selected  from  the 
series  of  Eclipse  photographs  brought  together  by  the  Royal  Astronomical  Society. 

Plate  17,  the  drawing  of  Donati's  Comet,  by  the  late  Professor  Bond,  is  from  the 
splendid  volume  of  observations  of  that  comet  in  the  Annals  of  the  Harvard  College 
Observatory. 

I  am  indebted  to  Professor  Barnard  for  the  use  of  the  comet  photographs  in  plate 
18,  which  are  selected  from  the  series  of  his  photographs  published  by  the  Royal 
Astronomical  Society.  I  owe  to  him  also  the  photograph  of  a  region  of  the  Milky  Way 
in  plate  81,  taken  from  the  same  series. 

For  the  permission  to  reproduce  the  two  comet  photographs  of  plate  19,  I  must 
thank  the  Astronomer  Royal  of  England  and  the  Astronomer  Royal  at  the  Cape, 
respectively. 

I  am  greatly  indebted  to  Professor  Hale,  Director  of  the  Yerkes  Observatory 
of  the  Uuiversity  of  Chicago,  and  to  Mr.  Ritchey,  Astronomer  at  that  observatory, 
who  took  the  photographs,  for  permission  to  use  the  three  photographs  of  the  Moon, 
plates  20,  21,  22  ;  the  photographs  of  the  nebulae  in  Orion  and  Andromeda,  plates  73 
and  47,  and  the  drawings  of  the  nebulae  round  Nova  Persei.  The  photographs  of  that 
star  and  of  the  region  in  which  it  appeared  I  owe  to  Mr.  Stanley  Williams,  of  Brighton. 

My  friend  Mr.  W.  E.  Wilson  kindly  allows  me  to  use  the  photographs  of  the 
Cluster  in  Hercules,  and  of  the  nebula  in  Cygnus,  forming  plate  75,  which  were  taken 
by  him  at  his  observatory  at  Daramona,  county  Westmeath.  To  Professor  Campbell, 
Director  of  the  Lick  Observatory,  I  owe  permission  to  reproduce  the  three  photo- 
graphs of  the  spiral  nebula  in  Canes  Venatici,  the  Ring  nebula  in  Lyra,  and  the  Dumb- 
bell nebula  in  Vulpecula,  made  with  the  Crossley  Reflector  by  his  lamented  pre- 
decessor, Professor  J.  E.  Keeler. 

Plate  80,  of  the  Pleiades  Cluster,  is  taken  from  a  photograph  by  the  brothers 
Henry,  published  in  a  report  of  the  Paris  Observatory  ;  and  for  the  photograph  of  the 
nebulae  in  that  cluster  I  am  indebted  to  the  late  Dr.  Isaac  Roberts,  of  Crowborough. 
Sussex. 

Plate  83,  illustrating  the  adoption  of  Standard  Time,  and  the  line  where  the 
date  changes,  has  been  made  from  information  kindly  furnished  by  the  Hydrographer 
to  the  Admiralty, 


PREFACE.  Vll. 

The  work  involved  has  been  very  onerous,  and  I  could  not  have  undertaken  it 
had  I  not  been  so  fortunate  as  to  have  had  the  aid  of  Mr.  Arthur  Hinks,  M.A.,  Chief 
Assistant  at  the  Cambridge  Observatory.  To  Mr.  Hinks  I  am  indebted  for  the 
selection  of  the  new  plates,  as  well  as  for  the  preparation  of  the  text  which 
accompanies  them.  I  would  like  to  record  my  thanks  to  him  for  all  his  skill 
and  zeal. 


CAMBRIDGE,  ROBERT  S.  BALL. 

November,  1904. 


CONTENTS. 


PAGE 

PREFACE v. — vii. 

CHAPTER  I. 
THE  CELESTIAL  SPHERE  AND  THE  SOLAR.  SYSTEM. 

The  Celestial  Sphere — The  Seasons — The  Horizon  and    the  Zenith — Refraction — 
~Diurnal  Parallax — Annual  Parallax— Apparent  Daily  R-otation^of  the  Heavensj 
J&ging  andJSfittijioj.— The  Signs  of  the  2odiac^-TIie~  Orbits  of  the  Inner  Planets—- 
The OrrTits  of  the  Outer  Planets— Bode's  Law '     ...       1-6 

CHAPTER  II. 
THE  PLANETS  AND  SATELLITES. 

The  Size  of  the  Planets— Phases  of  the  Planets  and  of  Saturn's  Rings— Systems  of 
Satellites— Map  of  Mars— Jupiter  and  Satellite  I. — Saturn 7-13 

CHAPTER  III. 
THE  SUN;    ECLIPSES  OF  THE  SUN  AND  MOON. 

The  Sun — Paths  of  Spots  Across  the  Sun's  Disc — Phases  of  the  Moon — Lunar  and 
Solar  Eclipses-Paths  of  Total  Eclipses  of  the  Sun  1901-1950— Typical  Solar  Corouse   15-19 

CHAPTER  IV. 
COMETS. 

Donati's  Comet— Holmes'  Cornet  and  the  Andromeda  Nebula— Comet  a  1893  IV. 
(Brooks)— Comet  1901  I.— Comet  b  1902  III.  (Perrine)  21-23 

CHAPTER  V. 

THE  MOON. 

The  Moon— Place  of  the  Moon— The  Lunar  Charts— Catalogue  of  Lunar  Objects  ...   25-33 


*•  CONTENTS. 

CHAPTER  VI. 

THE  SKY  MONTH  BY  MONTH  AND  THE  INDEX 
TO  THE  PLANETS. 

The  Monthly  Maps— Table  to  find  the  Aspect  of  the  Heavens  at  any  given  Month 
and  Hour  of  Night — Table  of  Planetary  Phenomena — Mercury— Venus— Index  to 
Venus — Mars — Index  to  Mars — Jupiter — Index  to  Jupiter — Saturn — Index  to 
Saturn— The  Naming  of  an  Unknown  Planet  35-48 

CHAPTER  VII. 
THE  STAR  MAPS. 

The  Maps — Variable  Stars — Algol  Variables— Short  Period  Variables — Irregularly 
Variable  Stars — Temporary  or  "New"  Stars — Meteor  Showers — Precession  ...  49-58 

CHAPTER  VIII. 
STAR  CLUSTERS  AND  NEBULA. 

The  Great  Nebula  in  Orion — The  Great  Nebula  in  Andromeda — The  Great  Star-Cluster 
in  Hercules  and  a  Nebula  in  Cygnus — The  Spiral  Nebula  in  Canes  Venatici — The 
Dumb-Bell  Nebula— The  Ring  Nebula  in  Lyra— The  Pleiades— The  Milky  Way 
around  the  Star  Cluster  Messier  11— Nova  Persei  and  the  Nebula  in  Motion  ...  59-64 

CHAPTER  IX. 
SELECT  LIST  OF  TELESCOPIC  OBJECTS. 

Distance  of  the  Stars — Distance  of  Star  in  Light  Years — Velocity  of  Star  at  Right 
Angles  to  the  Line  of  Sight— Brightness  of  Stars  compared  with  the  Sun — Masses 
of  the  Stars  compared  with  the  Sun — Spectroscopy — Spectroscopic  Binaries — Cata- 
logue and  description  of  objects  65-81 

APPENDIX — Standard  Time 82 

INDEX. 


LIST    OF    ILLUSTRATIONS. 


PLATE 

1.  THE  CELESTIAL  SPHERE 

2.  THE  SEASONS  AND  THE  TIDES 

3.  THE  INNER  PLANETS 

4.  THE  OUTER  PLANETS 

5.  THE  SIZE  OF  THE  PLANETS 

6.  PHASES  OF  THE  PLANETS  AND  OF  THE  RINGS  OF  SATURN 

7.  SYSTEMS  OF  SATELLITES 

8.  MAP  OF  MARS  1896-97.    A.  E.  DOUGLASS,  LOWELL  OBSERVATORY 

9.  JUPITER  AND  SATELLITE  I.    0.  LOHSE  ;    E.  E.  BARNARD  - 

10.  SATURN,  JULY  2ND,  1894.     E.  E.  BARNARD  - 

11.  A  GUEAT  SUN  SPOT,  1898,  SEPT.  HTH.    ROYAL  OBSERVATORY,  GREENWICH    - 

12.  (a.)  SOLAU  PROMINENCES  DRAWN  WITH  THE  SPECTROSCOPE  BY  J.  FENYI,  1895, 

JULY  15TH 

(6.)  SOLAR  PROMINENCES  PHOTOGRAPHED  BY  E.  E.  BARNARD   DURING  THE 
TOTAL  ECLIPSE  OF  THE  SUN,  1900  MAY  23RD 

13.  PATHS  OF  SPOTS  ACROSS  THE  SUN'S  Disc 

14.  ECLIPSES,  AND  PHASES  OF  THE  MOON 

15.  PATHS  OF  TOTAL  SOLAR  ECLIPSES,  1901-1950    - 

16.  THE  SOLAR  CORONA.    PHOTOGRAPHED  DURING  TOTAL  ECLIPSES  OF  THE  SUN  : 

(1)  1871,  DEC.  12TH— (2)  1882,  MAY  UTH— (3)  1893,  APRIL  16TH— (4)  1878, 
JULY  29tH-(5)  1889,  JAN.  IST— (6)  1900,  MAY  23RD      - 

17.  COMET  OF  DONATI,  OCT.  STH,  1858.  G.  P.  BOND,  HARVARD  COLLEGE  OBSERVATORY 

18.  HOLMES'  COMET  AND  THE  ANDROMEDA  NEBULA— BROOKS'  COMET,   1893,   IV. 

OCT.  20TH  AND  21ST.    E.  E.  BARNARD     - " 

19.  PERRINE'S  COMET,  1902,  III.  30m.  REFLECTOR,  ROYAL  OBSERVATORY,  GREENWICH 
GREAT  COMET  OF  1901,  MAY  4TH,  MCCLEAN  TELESCOPE,  ROYAL  OBSERVATORY, 

CAPE  OF  GOOD  HOPE 

20.  THE   MOON,  REGION  OF   THE    MARE    SERENITATIS,  G.  W.  RITCHEY,  40-iN. 

REFRACTOR,  YERK.ES  OBSERVATORY 
20A.  KEY  MAP  TO  DITTO     - 

21.  THE   MOON.    REGION   OF   CLAVIUS    AND   TYCHO,    G.   W.    RITCHEY,    40-iN. 

REFRACTOR,  YERKES  OBSERVATORY 
2lA.  KEY  MAP  TO  DITTO     • 

22.  THE  MOON.     REGION  OF  COPERNICUS,  G.  W.  RITCHEY,   40-m.   REFRACTOR, 

YERKES  OBSERVATORY 


To  follow  page  6 


To  follow  page  14. 


To  follow  page  20. 


To  follow  page  24 


To  follow  page  34 


Xll 


LIST  OF  ILLUSTRATIONS. 


22A.  KEY  MAP  TO  DITTO 

23.  CHART  OF  THE  MOON,  FIRST  QUADRANT 

24.  ,,  ,,       SECOND      „  -  • 

25-  .  „        THIRD        „  J-To  follow  page  34 

26.    t          ,,  ,,        FOURTH    ,, 

27-38.  THE  MOON,  SRD  DAY  TO  14TH  DAY 

27A-38A.  KEY  MAPS  TO  DITTO 

39-50.  MONTHLY  STAR  MAPS,  JANUARY  TO  DECEMBER  To  follow  page  48 

51-70.  GENERAL  STAR  MAPS,  SECTIONS  1-20     -  -    ~| 

71.  NORTHERN  INDEX  MAP       -  -  \-To  follow  page  53 

72.  SOUTHERN       ,,       ,, 

73.  THE   GREAT   NEBULA   IN   ORION,  G.  W.  RITCHEY,  2-FT.  REFLECTOR,  YERKES 

OBSERVATORY 

74.  THE  GREAT  NEBULA  IN  ANDROMEDA  (M.  31),  G.  \V.  RITCHEY,  2  FT.  REFLECTOR, 

YERKES  OBSERVATORY 

75.  PHOTOGRAPHS  TAKEN   WITH   24  IN.   REFLECTOR,   W.  E.  WILSON,  DARAMONA, 

IRELAND  :  (1)  THE  GLOBULAR  CLUSTER  IN  HERCULES— (2)  AN  IRREGULAR 
NEBULA  IN  CYGNUS 

76.  SPIRAL  NEBULA  IN  CANES  VENATICI  (M.  51),  J.  E.  KEELER,  CROSSLEY  REFLECTOR, 

LICK  OBSERVATORY 

77.  THE  RING  NEBULA  IN  LYRA  (M.  57),  J.  E.  KEELER,  CUOSSLEY  REFLECTOR,  LICK 

OBSERVATORY 

78.  THE  DUMB-BELL  NEBULA  IN  VULPECULA,  J.  E.  KEELER,  CROSSLEY  REFLECTOR, 

LICK  OBSERVATORY       -  To  follow  page  64 

79.  THE  NERJJL.E  IN  THE  PLEIADES,  ISAAC  ROBERTS,  20-iN.  REFLECTOR,  CROW- 

BOROUGH,  SUSSEX  - 

80.  THE  PLEIADES,  FROM  PHOTOGRAPH  BY  P.  AND  P.  HENRY,  PARIS  OBSERVATORY 
80A.  KEY  PLATE,  TO  DITTO  - 

81.  THE  MILKY  WAY  AROUND  THE  STAR  CLUSTER,  MESSIER  11,  E.  E.  BARNARD, 

6-iN.  PORTRAIT  LENS,  LICK  OBSERVATORY  - 

82.  NOVAPERSEI — PHOTOGRAPHS  BY  A.  STANLEY  WILLIAMS,  BRIGHTON.  (1)  BEFORE 

THE  APPEARANCE  OP  THE  NOVA  ;  (2)  THE  NOVA — TUB  MOVING  NEBULA  SUR- 
ROUNDING NOVA  PERSEI.     DRAWN  BY  G.  W.  RITCHEY  FROM  PHOTOGRAPHS 

TAKEN  WITH  24-IN.  REFLECTOR,  YBliKES  OBSERVATORY  :   (3)  1901,  SEPTEMBER 

20TH  ;  (4)  1901,  NOVEMBER  13TH 

83.  STANDARD  TIME    .  -  -  -  -  -  To  follow  page  82 


CHAPTER  1. 

PLATES  1  <fe  2. 
THE   CELESTIAL   SPHERE,   THE   SEASONS,   &c. 

Our  first  plate  is  an  attempt  to  represent  in  a  diagram  what  cannot  properly  be  figured, 
save  in  the  imagination  of  the  student,  the  relation  of  the  terrestrial  sphere  to  the  celestial. 
The  celestial  sphere  must  be  imar/ined  of  infinite  diameter ;  in  our  figure  it  has  to  be 
represented  as  only  twice  the  diameter  of  the  orbit  of  the  Earth  about  the  Sun. 

The  centre  of  the  Earth  describes  an  ellipse,  very  nearly  a  circle,  in  a  plane  which  passes 
through  the  centre  of  the  Sun.  If  this  plane  is  produced  all  ways  to  infinity  it  cuts  the 
celestial  sphere  in  a  great  circle — the  Ecliptic.  Could  the  Sun  be  viewed  among  the  stars  from 
the  centre  of  the  Earth,  it  would  be  seen  to  lie  always  upon  this  great  circle. 

The  axis  of  rotation  of  the  Earth  makes  a  constant  angle  with  the  plane  of  the  ecliptic, 
and  points  constantly  in  the  same  direction.  (We  are  neglecting  here  the  very  slow  effects  of 
precession).  Consequently  the  plane  of  the  Earth's  equator,  produced  all  ways  to  infinity, 
cuts  the  celestial  sphere  in  a  fixed  great  circle  which  is  the  celestial  equator,  and  the  axis  of 
rotation  of  the  Earth,  similarly  produced  to  infinity,  cuts  the  celestial  sphere,  in  two  points 
which  are  the  poles  of  the  celestial  equator,  more  commonly  called  the  poles  of  the  sky. 

It  is  now  easy  to  see  that  at  northern  midwinter  the  north  pole  of  the  Earth  is  turned 
away  from  the  Sun,  which  is  at  the  winter  solstice,  the  point  where  the  ecliptic  is  farthest 
south  of  the  celestial  equator.  As  spring  advances,  the  Sun,  apparently  moving  along  the 
ecliptic,  approaches  the  celestial  equator,  and  enters  it  at  the  vernal  equinox,  otherwise 
known  as  the  "  first  point  of  Aries."  At  northern  midsummer  the  north  pole  of  the  Earth 
is  turned  towards  the  Sun,  which  then  appears  farthest  north  of  the  celestial  equator,  and 
thence  forward  begins  to  dip  again  towards  the  autumnal  equinox. 

It  is  found  convenient  to  refer  the  places  of  all  stars  on  the  celestial  sphere  to  the 
celestial  pole  and  equator.  Distance  north  or  south  of  the  equator  is  called  north  or  south 
declination,  and  corresponds  to  north  or  south  latitude  of  a  place  on  Earth.  This  provides 
for  one  co-ordinate.  The  other,  called  Eight  Ascension  in  the  sky,  corresponds  to  longitude  on 
Earth ;  and  as  the  longitude  of  a  place  is  measured  from  a  meridian  on  the  Earth,  which  passes 
through  the  terrestrial  poles  and  through  an  arbitrary  point,  namely  Greenwich  Observatory, 
so  Right  Ascension  in  the  sky  is  measured  from  a  meridian  passing  through  the  poles  of  the 
celestial  equator  and  a  point,  the  vernal  equinox,  the  point  where  the  ecliptic  cuts  the 
equator  and  the  Sun  crosses  from  south  to  north. 

Right  Ascension  is  commonly  expressed  in  hours,  minutes,  and  seconds  of  time,  because  it 
is  measured  as  the  time  which  elapses  between  the  passage  over  the  meridian  of  any  place  of 


POPULAR  GUIDE  TO  THE  HEAVENS. 


the  first  point  of  Aries  and  of  the  object  whose  position  is  to  be  defined.  Declination  is 
expressed  in  degrees  of  arc,  because  it  is  usually  measured  by  graduated  circles  so  divided  on 
the  instrument. 

For  further  account  of  the  subject  of  the  measurement  of  positions  on  the  celestial  sphere, 
the  student  must  be  referred  to  the  text  books  of  spherical  astronomy. 

THE  HORIZON  AND  THE  ZENITH, 

An  observer  upon  the  Earth  is  debarred  by  the  Earth  itself  from  seeing  more  than  one 
half  the  sky  at  once.  The  plane  which  touches  the  Earth  at  the  point  where  he  stands, 
produced  all  ways  to  meet  the  celestial  sphere,  cuts  it  in  the  great  circle  which  is  his  celestial 
horizon.  When  the  observer  is  looking  over  the  sea,  his  visible  horizon  is,  owing  to  the 
spherical  shape  of  the  Earth,  depressed  below  the  above  defined  celestial  horizon  by  an 
amount  which  depends  on  the  height  of  the  observer  above  sea  level.  This  depression  is 
called  the  "dip  of  the  horizon,"  and  must  be  taken  into  account  when  altitudes  of  a  heavenly 
body  are  measured  from  the  visible  sea  horizon.  In  other  words,  the  celestial  horizon  is  90° 
from  the  zenith,  the  point  vertically  above  the  observer  ;  the  visible  sea  horizon  is  more  than 
90°,  and  the  observer  can  by  this  amount  see  more  than  half  the  sky. 

REFRACTION. 

The  effect  of  the  layers  of  air  through  which  light  must  pass  on  its  way  from  a  star  to 
the  observer  upon  the  Earth  is  to  raise  the  star  apparently  above  its  real  position.  The  effect 
of  refraction  is  sometimes  very  obvious  when  either  the  Sun  or  Moon  is  close  to  the  horizon, 
in  a  flattening  of  the  solar  or  lunar  disc.  The  closer  a  body  to  the  horizon,  the  more  it  is 
raised  by  refraction  ;  the  lower  limb  of  the  Sun  is  consequently  raised  more  than  the  upper, 
with  the  result  that  the  Sun  appears  no  longer  round,  but  flattened. 

On  the  horizon  a  body  is  apparently  raised  34'  of  an  arc. 
At  an  elevation  of  1° 
3° 

5° 

10° 

20° 

„  30° 
„  40° 

Above  40°  of  elevation  the  refraction  is  measured  by  seconds  of  arc  alone,  and  becomes 
less  and  less  till  it  vanishes  at  the  zenith. 

DIURNAL  PARALLAX. 

It  is  clear  that  when  a  celestial  body  is  viewed  from  a  point  on  the  Earth's  surface,  it 
cannot  appear  precisely  in  the  same  direction  as  when  it  is  viewed  from  the  centre.  The 
difference  is  named  Diurnal  Parallax.  Owing  to  the  small  size  of  the  Earth,  and  the  vast 
distance  of  the  stars,  the  Diurnal  Parallax  of  stars  is  absolutely  insensible.  With  the  Sun 
and  planets,  however,  the  case  is  different.  The  nearer  the  body  to  the  Earth,  and  the 
nearer  to  the  horizon  of  the  observer,  the  greater  the  effect.  Upon  the  Moon  the  effect  is 
quite  large  :  when  the  Moon  is  just  rising  it  appears  lower  among  the  stars  than  it  would  do 
from  the  centre  of  the  Earth,  by  nearly  a  degree,  or  about  two  diameters  of  the  Moon. 


PARALLA.X.  3 

Owing  to  this  large  displacement  in  the  position  of  the  Moon  as  seen  from  different  parts  of 
the  Earth,  a  star  which  is  occulted  by  the  Moon  at  one  place  may  be  completely  clear  of  it  at 
another,  and,  similarly,  while  at  one  place  a  total  eclipse  of  the  Sun  is  seen,  at  another  the 
eclipse  may  not  even  be  partial. 

ANNUAL  PARALLAX. 

It  is  equally  clear  from  the  figure  that  a  star  will  not  be  seen  exactly  in  the  same 
direction  at  different  times  of  year.  By  far  the  greater  number  of  stars  are  so  far  away  that 
not  even  the  displacement  of  the  Earth  by  186,000,000  miles  produces  any  sensible  effect. 
But  upon  a  certain  number  of  nearer  stars  the  effect  is  just  measurable.  They  shift  their 
positions  relative  to  the  more  distant  stars  very  slightly  during  the  year ;  and  this  effect  is 
called  Annual  Parallax. 

APPARENT    DAILY   ROTATION    OF   THE    HEAVENS: 
RISING  AND  SETTING. 

In  consequence  of  the  actual  rotation  of  the  Earth,  from  W.  to  E.,  an  observer  upon  it 
sees  the  heavens  apparently  rotating  about  an  axis  directed  to  the  pole  of  the  sky.  Every 
celestial  body  therefore  apparently  describes  once  a  day  a  circle  round  the  pole.  The  pole  is 
elevated  above  the  horizon  by  an  amount  equal  to  the  latitude  of  the  observer.  Suppose  this 
is  50°.  A  star  nearer  the  pole  than  50°  describes  the  whole  of  its  circle  above  the  horizon  ; 
it  never  sets,  and  is  called  circumpolar.  A  star  further  than  50°  from  the  pole  will  not  be 
circumpolar  ;  part  of  its  daily  circle  will  be  below  the  horizon,  and  a  greater  part  the  farther 
the  star  is  from  the  pole,  until  for  a  star  180° — 50°,  i.e.,  130°  from  the  pole,  the  whole  circle  is 
permanently  below  the  horizon  ;  and  in  lat.  50°  N.  this  star  will  never  rise  at  all. 

The  figure  in  Plate  2,  "  apparent  diurnal  paths  of  the  Sun,"  illustrates  this  principle  in 
the  case  of  the  Sun,  and  shows  why  the  days  are  longer  in  summer  than  in  winter  :  the  sun 
is  nearer  the  north  pole  of  the  sky  in  summer  than  in  winter,  for  we  have  seen  that  the 
ecliptic  cuts  the  equator  at  a  considerable  angle  (23°  27'). 

Tides. — The  figure  in  the  lower  part  of  Plate  2  illustrates  the  fact  that  when  a  body  like 
the  Earth,  covered  with  an  ocean,  is  rotating,  the  attraction  of  the  Sun  or  Moon  produces 
disturbances  in  the  level  of  the  ocean,  which  are  called  tides.  Though  these  protuberances 
are  caused  by  the  attraction  of  the  tide  producing  body  they  are  not  necessarily,  nor  indeed 
generally,  in  line  with  it.  On  an  earth  whose  ocean  is  much  broken  up  by  continents  the 
tidal  waves  are  much  broken  up  and  disturbed,  and  the  theory  of  the  tides  becomes 
exceedingly  complex.  Owing  to  its  relative  nearness  more  than  counter-balancing  its 
smallness,  the  Moon  is  a  much  more  efficient  tide  producing  agent  than  the  Sun. 

The  figure  at  the  top  of  the  Plate  is  to  explain  the  expressions  spring  and  neap  tides. 
At  New  Moon  and  Full  Moon  the  tides  raised  separately  by  the  Sun  and  Moon  conspire,  and 
an  exceptionally  high  tide  is  produced,  which  is  called  a  spring  tide.  At  first  or  last  quarter 
of  the  Moon,  the  Sun  tends  to  produce  low  water  when  and  where  the  Moon  tends  to 
produce  high  water,  and  the  result  is  a  small  or  neap  tide. 

THE  SIGNS  OF  THE  ZODIAC. 

The  region  of  the  heavens  along  the  ecliptic,  or  the  zodiac,  was  divided  by  the  ancients 
into  twelve  parts,  or  signs,  each  30°  in  length,  which  took  their  names  from  the  principal 


POPULAR   GUIDE   TO   THE   HEAVENS. 


constellations  along  the  zodiac.  Thus,  starting  from  the  Vernal  Equinox,  the  first  sign  was 
called  Aries,  the  second  Taurus,  and  so  on.  The  gradual  change  in  the  position  of  the 
Equator  and  Equinox,  due  to  precession,  has  thrown  back  the  Equinox  into  the  constellation 
Pisces,  and  displaced  all  the  signs  of  the  zodiac  from  the  constellations  whose  names  they 
bear.  Nevertheless  the  old  names  are  retained,  and  the  student  must  be  warned  against 
possible  confusion.  When  the  Almanac  says  "  Sun  enters  Aries ;  spring  commences,"  a 
reference  to  the  star  maps  will  show  that  the  Sun  is  still  in  Pisces,  and  will  .be  for  a  month 
We  must  distinguish,  therefore,  the  names  of  the  constellations  from  the  same  names  applied 
to  the  signs  of  the  zodiac. 

The  Signs  of  the  Zodiac,  with  their  symbols,  are  given  in  the  following  table.  Counting 
from  the  Vernal  Equinox  we  have — 

0°  to  30°  ...  0.  T   Aries.  180°  to  210°  ...      VI.  =&  Libra. 

30°  to  60°  ...  I.  «    Taurus.  210°  to  240°  ...    VII.   m.  Scorpio. 

60°  to  90°  ...  II.  n  Gemini.  240°  to  270'  ...VIII.    J    Sagittarius. 

90°  to  120°  ...  III.  55  Cancer.  270°  to  300°  ...      IX.  >y  Capricornus. 

20°  to  150°  ...  IV.  Si  Leo.  300°  to  330°  ...       X.  ^  Aquarius. 

50°  to  180°  ...  V.  "B  Virgo.  330°  to  360°  ...     XL    X   Pisces. 

PLATE    3. 

THE  ORBITS  OF  THE  INNER  PLANETS. 

In  the  attempt  to  represent  the  orbits  of  celestial  bodies  on  maps  or  charts,  it  must 
always  be  remembered  that,  except  in  the  case  of  orbits  which  happen  to  lie  in  the  same  plane 
it  is  impossible  to  depict  on  any  drawing  the  veritable  position  of  more  than  one.  We  are 
obliged  to  resort  to  some  process  of  a  more  or  less  artificial  character.  For  instance,  we  take 
the  plane  of  the  Ecliptic,  that  is,  the  Earth's  orbit,  as  the  plane  of  the  paper,  and  then  we 
simply  lay  down  on  it  the  orbits  of  the  other  bodies,  notwithstanding  that  their  planes  are 
inclined  to  the  Ecliptic.  The  points  in  which  the  real  orbit  passes  through  the  plane  of 
representation  are  called  the  Nodes,  the  ascending  node  being  that  at  which  the  planet  passes 
from  the  southerly  to  the  northerly  side  of  the  plane.  Each  orbit  may  be  conceived  to  be 
turned  around  its  line  of  nodes  till  its  plane  coincides  with  the  Ecliptic.  It  is  thus  tha 
Plate  III.  is  produced. 

The  path  which  every  planet 
describes  is  an  ellipse,  and  the 
Sun  is  situated  in  one  of  the  two 
foci  of  the  ellipse,  S  or  H.  The 
longest  diameter  of  the  ellipse, 
P  A,  which  passes  through  the 
two  foci,  is  called  the  major 
axis;  the  diameter  X  Y,  at 
right  angles  to  it,  is  the  minor 
axis,  and  the  two  intersect  in 
the  centre,  0,  of  the  ellipse. 
The  points  A  and  P  are  the 
Apsides  of  the  ellipse.  We  will 
suppose  the  Sun  is  in  the  focus 


Y 

TEE  ELLIPSH. 


THE  CEBITS   OF   THE   INNER   PLANETS.  5 

S  in  the  figure,  and  that  a  planet  is  describing,  under  its  attraction,  the  ellipse. 
The  ellipse  is  called  its  orbit.  The  point  P,  nearest  to  the  Sun,  is  the  Perihelion 
of  the  orbit ;  the  point  A,  farthest  away,  is  the  Aphelion.  The  half  major  axis,  0  P,  which 
is  also  equal  to  the  distance  S  X,  is  called  the  mean  distance.  The  ratio  of  O  S  to  O  P  is  the 
eccentricity  of  the  orbit ;  the  smaller  this  ratio  is,  the  more  does  the  ellipse  resemble  a  circle. 
The  orbits  of  the  more  important  planets  all  have  small  eccentricities. 

It  will  be  convenient  to  give  here  the  symbols  which  are  in  use  for  the  Sun,  Moon,  and 
Planets,  and  certain  other  signs  used  occasionally  in  this  work. 

Explanation  of  Astronomical  Symbols  and  Abbreviations. 


O     The  Sun. 
<i      The  Moon. 
£     Mercury. 
9     Venus, 
ffi  or  6  The  Earth. 

h     Hours. 

m    Minutes  of  Time. 

8     Seconds  of  Time. 


cS  Mars. 

H  Jupiter, 

h  Saturn, 

y  Uranus. 

*f  Neptune. 

°     Decrees. 


Minutes  of  Arc. 
Seconds  of  Arc. 


6  Conjunction. 

O.  Quadrature. 

<?  •  Opposition. 

ft  Ascending  Node. 

y  Descending  Node. 

N.     North.        S.     South. 
E.     East.  W.     West. 


The  orbits  of  the  planets  Mercury,  Venus,  Earth,  Eros,  and  Mars  are  represented  in  this 
Plate,  and  for  illustrating  the  use  of  it  we  take  the  orbit  of  Mercury.  The  point  A  is  the 
Aphelion  where  the  planet  is  most  distant  from  the  Sun.  The  next  point  marked  is  the 
Ascending  Node  ft,  where  the  orbit  comes  through  the  plane  of  the  paper,  the  inclination 
being  7"  0',  as  given  in  the  table  in  the  upper  right  hand  corner  of  the  map.  P  is  the 
Perihelion,  where  Mercury  is  nearest  the  Sun.  For  a  complete  revolution  this  planet  requires 
a  period  of  87 '969  days.  Similar  remarks  apply  to  the  other  orbits.  Thus,  for  instance 
Mars,  the  outermost  of  the  four  planets  shown  in  this  figure,  revolves  in  the  period  of  686'951 
days.  Its  Perihelion  is  marked  P,  and  Aphelion  A,  the  Ascending  Node  is  Si,  and  the 
inclination  is  1°  51'.  The  inclinations  of  the  cometary  orbits  are  given  in  the  right  hand 
lower  corner  of  the  Plate.  The  orbits  of  the  three  following  comets  are  drawn,  Biela's  Comet, 
Comet  I.  1866,  and  Comet  III,  1862.  These  have  been  chosen  because  they  possess  the 
additional  interest  of  being  the  paths  of  the  three  chief  meteor  swarms.  The  famous  showers 
of  "  Leonids,"  which  used  to  appear  about  November  13th,  in  magnificent  displays  every  33 
years,  move  in  the  track  of  Comet  I.  1866.  The  "  Andromedids,"  or  meteors  of  November 
27th,  have  the  same  orbit  as  Biela's  Comet,  and  the  "  Perseids  "  pursue  the  course  of  Comet 
III.  1862.  In  the  case  of  each  of  the  cometary  orbits  the  Descending  Node  has  been  marked 
on  the  Plate,  as  it  is  at  this  Node  that  the  Earth  meets  the  associated  meteor  swarm. 

The  planet  Eros  is  a  recently  discovered  small  planet,  whose  orbit  is  of  a  remarkable 
character.  It  is  very  eccentric,  and  considerably  inclined  to  the  ecliptic  ;  and  on  favourable 
occasions  the  planet  may  be  within  about  13,000,000  miles  of  the  Earth,  nearer  to  us  than 
any  celestial  body  except  our  own  Satellite. 


PLATE   4. 
THE  ORBITS  OF  THE  OUTER  PLANETS, 

The  innermost  orbit  on  this  Plate  is  that  of  Mars,  for  those  belonging  to  planets  still 
closer  to  the  Sun  would  be  too  small  to  be  shown  in  a  figure  of  the  scale  necessary  for  the 
outer  planets. 


POPULAR  GUIDE   TO   THE   HEAVENS. 


Next  to  Mars  comes  the  zone  of  minor  planets,  of  which  more  than  500  are  now  known, 
while  some  twenty  or  thirty  new  ones  are  discovered  by  photography  yearly.  Their  orbits 
are  tangled  in  a  way  impossible  to  represent  otherwise  than  conventionally  in  a  figure  of 
small  size.  They  exhibit  great  diversity  of  eccentricity  and  inclination  to  the  ecliptic.  And 
the  task  of  keeping  up  the  computation  of  the  places  of  this  fast  growing  family  of  tiny 
planets  is  becoming  almost  beyond  reasonable  possibility.  In  our  figure  the  innermost  repre- 
sented is  Medusa,  with  a  period  of  312  years  ;  the  outermost  Hilda,  with  a  period  of  V90.  A 
later  discovery,  Adalberta,  lies  closer  to  the  Sun  than  Medusa,  with  a  period  of  3'01  years  ; 
and  Thule,  with  a  period  of  8'86  years,  lies  considerably  farther  out  than  Hilda. 

Beyond  the  zone  of  minor  planets  lie  the  major  planets  Jupiter,  Saturn,  Uranus,  and 
Neptune. 

On  this  Plate  are  also  represented  the  orbits  of  several  interesting  comets  belonging  to  the 
Solar  System.  The  Comet  of  Encke  has  the  smallest  orbit  of  any  known  comet,  and  it  is 
especially  interesting  because  of  the  somewhat  irregular  and  unexplained  acceleration  to  which 
it  is  subject.  There  is  room  on  this  Plate  to  show  the  orbit  of  Biela's  Comet,  part  of  which 
was  shown  on  the  preceeding  Plate. 

Halley's  Comet  is  the  only  periodic  comet  which  makes  a  splendid  appearance  ;  the 
others  are  all  small,  many  of  them  almost  insignificant  objects.  But  Halley's  Comet  on  its 
last  return  had  a  nucleus  as  bright  as  a  first  magnitude  star,  and  a  tail  twenty-five  degrees 
long ;  and  its  next  return  in  1910  will  be  awaited  with  very  great  interest. 

A  portion  of  the  orbit  of  the  Comet  of  1882  is  shown  on  account  of  its  remarkable  nature. 
The  comet  passed  so  close  to  the  Sun  that  it  almost  grazed,  and  it  swung  round  180°  of  its 
orbit  in  the  space  of  three  hours. 

A  remarkable  relation,  hitherto  unexplained,  connects  the  distances  of  the  various  planets 
from  the  Sun. 

If  we  write  down  the  series  of  numbers — 

0         3         6         12         24         48         96         192         384, 
and  add  4  to  each,  we  have 

4         7         10         16         28         52         100         196         388. 

The  first  four  are  very  nearly  in  the  proportion  of  the  distances  from  the  Sun  of  Mercury, 
Venus,  the  Earth,  and  Mars  ;  52  and  100  represent  equally  well  the  distances  of  Jupiter  and 
Saturn  ;  the  intermediate  No.  28,  which  stands  for  the  average  minor  planet,  actually  suggested 
the  search  for  them  ;  when  Uranus  was  discovered,  it  was  found  to  fit  196  ;  and  when 
there  was  a  suspicion  of  a  planet  beyond  Uranus,  it  was  assumed  that  its  distance  would  be 
nearly  represented  by  388.  But  it  is  not ;  300  is  the  real  number,  and  here  the  rule,  which 
is  called  Bode's  law,  breaks  down  completely. 


. 


1-VERSiTV 


^jmARv" 

'OF  THE 

tVERSlTY 

or 


(7) 
CHAPTER  II. 


PLATE  5. 
THE  SIZE   OF  THE   PLANETS. 

Plate  5  has  been  drawn  to  show  the  relation  and  actual  sizes  of  the  different  planets  and 
of  the  rings  of  Saturn.  The  determination  of  the  size  of  a  planet  in  miles  is  by  no  means  a 
simple  matter.  Firstly,  it  is  necessary  to  measure  the  angular  size  of  the  planets,  as  seen  from 
the  Earth,  in  seconds  of  arc,  and  this  is  affected  by  the  phenomenon  of  irradiation  by  which 
a  bright  object  against  a  dark  background  looks  larger  than  it  really  is.  Also  the  planets 
Uranus  and  Neptune  are  so  far  away  and  look  so  small  that  it  is  hard  to  measure  them,  and 
it  is  still  quite  uncertain  which  is  really  the  larger.  We  have  therefore  shown  them  equal. 
When  the  angular  diameters  at  a  given  distance  have  been  found,  we  next  require  a  knowledge 
of  the  Solar  Parallax,  that  is,  of  the  angular  radius  of  the  Earth  as  seen  from  the  Sun  ;  the 
adopted  value  of  this  quantity  is  8"80.  And,  finally,  we  require  the  diameter  of  the  Earth  in 
miles  ;  for  this  we  adopt  Col.  Clarke's  latest  value,  7926'6  miles. 

We  can  then  calculate  the  diameters  of  the  planets  given  below,  and  shown  to  scale  upon 
the  Plate.  The  times  of  axial  rotation  and  the  inclination  of  the  planets'  equator  to  the 
Ecliptic  are  given  when  they  are  known. 


Diameter 

in  miles. 

Mercury 

3,000 

Venus 

7,700 

Earth 

7,926 

Mars 

4,300 

Jupiter 

/  Equatorial 
\  Polar 

90,000 
84,000 

Saturn 

i  Equatorial 
\  Polar 

76,000 
70,000 

Uranus 

31,000 

Neptune 

31,000 

Time  of  Rotation. 

Unknown 
Unknown 


23h- 

24 
9 


37 


4"- 
23 


10      14       0 

Unknown 
Unknown 


Inclination  of  Equator 
to  Ecliptic. 

Unknown 
Unknown 
23*   27' 
26     21 
3       5 

28     10 

Unknown 
Unknown 


Dimensions  of  Saturn's  rings  : 

Outer  radius  of  outer  ring  ...  ...  ...  86,000  miles. 

Inner  radius  of  outer  ring  ...  ...  ...  75,000     „ 

Outer  radius  of  inner  ring  -  ...  ...  ...  73,000     „ 

Inner  radius  of  inner  ring  ...  ...  ...  55,000     „ 

Inner  radius  of  dusky  ring  ...  ...  ...  44,000     „ 

All  these  dimensions  are  based  upon  a  careful  comparison  of  the  most  recent  measures 
made  with  the  great  telescopes  at  Lick,  Yerkes,  Washington,  and  Greenwich  Observatories. 
The  time  of  rotation  and  position  of  the  equator  of  Mercury,  Venus,  Uranus,  and  Neptune 
are  unknown  because  of  the  want  of  definite  markings  on  those  planets.  There  is  consider- 
able reason  to  suppose  that  the  periods  of  rotation  of  Mercury  and  Venus  are  the  same  as 
their  periods  of  revolution  round  the  Sun,  namely  88*0  and  224'7  days.  In  that  case  they 
would  always  turn  the  same  face  towards  the  Sun.  If  we  assume,  which  is  probably  true, 
that  the  satellites  of  Uranus  and  Neptune  move  nearly  in  the  planes  of  the  planets'  equators, 
then  we  may  add  to  the  last  column  for  those  planets,  101°  and  145°,  the  inclinations  being 
given  greater  than  90°  to  conform  to  the  fact  that  the  satellites,  unlike  any  other  bodies 
in  the  Solar  System,  move  in  a  retrograde  direction. 


POPULAR  GUIDE   TO   THE   HEAVENS. 

Recent  measures  have  given  the  following  diameters  for  the  four  principal  minor  planets 
Ceres  ...  ...  ...        480  miles. 

Pallas  ...  ...  ...         300      „ 

Juno  ...  ...  ...         120      „ 

Vesta  240 


PLATE   6. 
PHASES  OF  THE  PLANETS  AND  OF  SATURN'S  RINGS. 

This  Plate  exhibits  the  appearances  of  the  planets  Mars,  Venus,  and  Saturn  when 
occupying  different  parts  of  their  orbits.  A  reference  to  Plate  3  makes  it  clear  that  the  distance 
between  the  Earth  and  Mars  must  vary  considerably  at  different  dates,  according  to  the  positions 
which  the  bodies  occupy  in  their  paths  around  the  Sun.  Of  course,  if  the  orbits  were  both 
circular,  it  is  clear  that  the  greatest  possible  separation  between  the  two  bodies  would  be 
attained  at  every  conjunction  ;  that  is  to  say,  whenever  the  Earth,  Sun,  and  Planet  are  in  a 
straight  line  (at  least,  in  their  projected  orbits),  the  Earth  and  Planet  being  at  opposite  sides 
of  the  Sun.  The  same  diagram  makes  it  plain  that  the  least  distance  apart  would  occur  at 
every  opposition;  that  is,  whenever  the  three  bodies,  as  represented  in  their  projected  orbits, 
were  in  a  straight  line,  with  the  Earth  in  the  middle. 

The  eccentricity  of  the  orbit  of  Mars  considerably  modifies  the  circumstances.  It  will  be 
seen,  by  referring  to  Plate  3,  that  an  opposition  occurring  in  the  latter  half  of  the  year  will 
generally  be  more  i'avourable  (i.e.,  bring  the  two  bodies  closer  together)  than  one  in  the  first 
half  of  the  year,  and  that  the  most  favourable  opposition  happens  when  the  Earth  and  Planet 
are  situated  in  about  333°  longitude  On  the  other  hand,  an  opposition  occurring  in 
longitude  153°  will  be  as  unfavourable  as  possible.  The  Earth's  longitude  on  August  26th  is 
333°,  and  on  February  22nd  it  is  153°  ;  hence  the  most  favourable  opposition  of  Mars  will 
occur  on  August  26th,  and  the  closer  to  that  date  the  opposition  happens  the  better.  The  most 
unsuitable  oppositions  are  about  February  22nd. 

The  greatest  distance  at  which  the  two  planets  can  possibly  be  separated  is  attained 
when  the  Earth's  longitude  is  333°,  and  that  of  Mars  153° ;  that  is  to  say,  when  conjunction 
occurs  about  August  26th. 

Figures  1,  4,  and  5,  in  the  upper  part  of  the  left-hand  portion  of  Plate  6,  show  the 
relative  apparent  sizes  of  the  planet— at  most  favourable  opposition  (August  26th),  at  least 
favourable  opposition  (February  22nd),  and  at  its  greatest  possible  distance.  These  views 
illustrate  the  advantage  of  an  opposition  occurring  somewhere  near  the  end  of  August,  when 
the  appearance  of  the  planet  is  to  be  studied. 

When  the  lines  from  the  Sun  to  the  Earth  and  the  Sun  to  the  Planet  are  at  right  angles, 
the  Planet  is  said  to  be  in  quadrature.  A  very  distinct  phase  is  then  perceptible  in  Mars,  by 
which  about  a  quarter  of  its  diameter  is  cut  off.  The  appearances  of  the  planet  at  western 
and  eastern  quadrature,  as  shown  in  an  inverting  telescope,  and  the  apparent  size  of  the 
planet  on  the  same  scale  as  the  other  figures,  is  also  given.  For  the  topography  of  the  planet, 
the  reader  may  refer  to  Plate  8.  As  to  the  times  and  seasons  for  observing  Mars  in  its  vary 
ing  aspects,  reference  may  be  made  to  the  Index  to  Planets,  see  pages  38  and  42. 

Since  the  orbit  of  Venus  lies  inside  that  of  the  Earth,  the  appearances  of  this  planet 
differ  considerably  from  those  of  an  exterior  planet  like  Mars.  It  is  obvious  that  the  nearest 
approach  of  the  two  bodies  will  occur  at  inferior  conjunction,  or  when  Venus  and  the  Earth 


PHASES   OF   THE    PLANETS   AND   OF  SATURN'S   RINGS.  9 

are  on  the  same  side  of  the  Sun ;  and  that  the  greatest  distance  between  them  will  occur  at 
superior  conjunction,  or  when  the  two  bodies  are  at  opposite  sides  of  the  Sun.  It  might,  at 
first  sight,  therefore,  be  supposed  that  at  inferior  conjunction  the  planet  would  be  seen  best, 
being  then  apparently  largest ;  and  that  it  would  be  least  favourably  placed  at  superior  con- 
junction. The  relative  apparent  sizes  of  this  planet  just  before  inferior,  and  at  superior,  con- 
junction are  shown  in  the  lower  part  of  the  left-hand  portion  of  this  plate  ;  but  since,  in  the 
former  configuration,  the  illuminated  part  of  the  globe  is  reduced  to  a  very  thin  crescent,  and 
since  in  both  cases  the  planet  Is  enveloped  in  the  Sun's  rays,  in  neither  of  these  phases  is  it 
suitably  situated  for  observation. 

Venus  attains  its  greatest  brightness  as  an  evening  star  about  a  month  after  its  greatest 
elongation  east.  The  greatest  brightness  of  the  same  planet  as  a  morning  star  precedes  by 
about  a  month  its  greatest  elongation  west. 

The  second  figure  has  been  drawn  to  represent  the  size  and  shape  of  Venus  when  most 
brilliant.  The  third  figure  exhibits  the  appearance  of  Venus  when  situated  at  a  distance  of  40° 
from  the  Sun  in  the  further  part  of  its  orbit.  In  this  position  it  presents  a  gibbous  form. 
It  will  be  seen,  however,  that  the  diminution  of  light  caused  by  its  increased  distance  from  the 
Earth,  more  than  compensates  for  the  larger  proportion  of  the  illuminated  surface  visible,  so 
that,  on  the  whole,  the  amount  of  light  received  from  the  planet  is  less  than  when  it  is  in  the 
position  corresponding  to  Figure  2.  In  the  Index  to  Planets,  p.  39,  the  method  of  finding  the 
position  of  Venus  for  any  date  up  to  1950  is  explained. 

For  the  general  details  of  the  planet  Saturn  reference  may  be  made  to  Plate  11.  In  this 
place  we  discuss  only  the  varying  appearances  of  the  rings.  The  right-hand  portion  of  Plate  6 
contains  twelve  figures  depicting  the  different  aspects  which  the  ringed  planet  presents 
according  to  the  position  it  happens  to  occupy  in  its  orbit.  In  connection  with  the  Table  of 
Planetary  Phenomena,  p.  38,  this  plate  will  enable  the  reader  to  determine  with  considerable 
accuracy  the  appearance  of  the  rings  at  any  time.  If  the  opposition  of  Saturn  occurs  in  the 
middle  cf  January  in  any  year,  it  will  be  found  that  Fig.  1  represents  the  system.  The  rings 
are  then  opened  nearly  to  their  full  extent,  and  the  upper  portion  of  the  ball  just  extends  be- 
yond the  outer  margin  of  the  rings.  If  the  opposition  occurs  in  February,  the  rings  will  be 
found  to  have  closed  up  somewhat,  and  to  appear  as  shown  in  Fig.  2.  If  the  opposition 
occurs  in  March,  the  rings  will  shrink  almost  to  a  straight  line,  as  in  Fig.  3.  At  oppositions 
occurring  in  April,  May,  and  June,  the  appearances  will  be  as  in  Figs.  4,  5,  and  6,  the  rings 
appearing  the  more  open  the  more  nearly  the  date  of  opposition  approaches  June. 
Figs.  7 — 12,  in  a  similar  way,  show  the  changes  which  this  system  will  undergo  at  oppositions 
occurring  in  the  latter  six  months  of  the  year. 

It  must,  of  course,  be  understood  that  the  appearance  here  depicted  for  any  month  will  not 
recur  every  year  in  that  month,  but  will  only  be  seen  in  those  years  in  which  the  opposition  of 
the  planet  occurs  during  the  month  in  question,  and  then  only  with  accuracy  at  the  date  of 
opposition.  But,  as  Saturn  takes  a  period  of  no  less  than  29|  years  to  accomplish  its  revolution, 
the  alteration  in  its  appearance  will  vary  very  little  for  several  months  before  and  after  opposi- 
tion, so  that  the  figure  for  any  month  may  be  taken  to  represent  the  appearance  of  the  system 
during  the  year  in  which  opposition  occurs  in  that  month.  Thus,  in  the  year  1921,  the  Table 
of  Planetary  Phenomena  tells  us  that  the  opposition  of  Saturn  takes  place  in  March,  whence 
we  learn  that  during  this  year  the  rings  will  be  almost  edgewise  towards  us.  Again,  in  the 
year  1928,  opposition  occurs  in  June,  from  which  we  infer  that  during  that  year  the  rings  will 
be  open  to  their  fullest  extent,  and  most  favourably  situated  for  observations 


10  POPULAR  GUIDE   TO   THE   HEAVENS. 

These  pictures  have,  as  usual,  been  drawn  to  represent  the  planet  as  seen  in  an  astiunomical 
telescope,  which  always  inverts  the  object,  so  that  Figs.  3 — 8  exhibit  the  appearance  of  the 
system  when  the  northern  face  of  the  ring  is  tilted  towards  us  so  as  to  become  visible,  while 
in  Figs  1  and  2,  and  9 — 12,  it  is  the  southern  side  of  the  rings  which  is  seen. 

To  facilitate  reference,  a  column  has  been  added  to  the  Table  of  Planetary  Phenomena, 
p.  38,  to  show  which  of  the  phases  are  presented  in  the  corresponding  opposition.  For 
example,  if  the  opposition  is  in  October,  the  column  alluded  to  gives  the  number  10,  which 
means  that  during  the  year  in  question  the  planet  Saturn  will  present,  when  visible  at  all,  a 
phase  resembling  that  shown  in  Fig.  10  on  Plate  8. 

At  the  times  when  the  ring  is  seen  edgewise  the  sequence  of  appearances  may  be  very 

complicated.    The  ring  may  become  quite  or  very  nearly  invisible  from  any  one  of  three  causes  ; 

(1.)  The  plane  of  the  ring  may  pass  through  the  Earth,  in  which  case  the  ring  is  seen 

exactly  edgewise.     And  as  the  ring  is  very  thin,  its  illuminated  edge  is  not  bright 

enough  to  be  seen,  and  the  ring  disappears  completely  in  all  telescopes. 

(2.)  The  plane  of  the  ring  may  pass  between  the  Earth  and  the  Sun,  in  which  case  the 

Sun  is  shining  on  the  opposite  side  of  the  ring  to  that  which  is  presented,  very 

obliquely,  to  the  Earth  ;  and  in  this  case  the  ring  is  almost  invisible,  even  in  great 

telescopes. 

(3.)  The  plane  of  the  ring  may  pass  through  the  Sun,  in  which  case  neither  side  of  the 

ring  is  effectively  illuminated,  and  again  it  almost  disappears. 

It  is  sometimes  a  matter  of  discussion  how  big  a  planet  looks  in  a  telescope  of  a  given 
magnifying  power.  By  means  of  this  Plate  the  question  may  be  answered.  The  Plate  is 
drawn  to  such  a  scale  that  if  it  is  placed  ten  feet  from  the  eye  the  figures  of  the  planetary 
discs  subtend  the  same  angle  as  the  planets'  images  themselves,  at  the  corresponding  phases 
would  do  when  seen  in  a  telescope  which  gives  a  magnifying  power  of  80  diameters. 


PLATE   7. 
SYSTEMS  OF  SATELLITES. 

This  Plate  exhibits  the  relative  dimensions  of  the  orbits  of  the  systems  of  satellites 
attending  certain  of  the  planets.  With  the  exception  of  the  system  surrounding  Mars,  which 
is  on  a  scale  twenty  times  as  large  as  the  rest,  the  orbits  are  all  laid  down  on  a  uniform  scale  of 
half  a  million  miles  to  the  inch.  The  periods  of  revolution  of  the  satellites  around  their 
primaries  are  also  marked  on  the  orbits  approximately.  More  complete  numerical  information 
than  it  has  been  found  convenient  to  represent  on  the  map  is  given  in  the  following  tables. 

THE  SATELLITE  OF  THE  EARTH  :  THE  MOON. 

Mean  distance  from  the  centre  of  the  Earth :  239,000  miles. 
Periodic  time  :  27  days,  7  hrs.  43  mins.  11  sees. 

m         n  -.  r  Mean  distance  from  Periodic  Time. 

THE  SATELLITES  OF  MARS  :      Centre  of  Planet.  Days      hrs.      mins.      sees : 

Phobos            5,850  miles  0  7  39  14 

Deimos            14,650      „  1  6  17  55 

THE  SATELLITES  OF  JUPITER  : 

V.  (Nameless)           ...       112,500  0  11  57  23 


I.  (lo) 261,000 

II.  (Europa) 415,000 

III.  (Ganymede)       ...  664,000 

IV.  (Callisto) 1,167,000 


1  18  27  34 

3  13  13  42 

7  3  42  33 

16  16  32  11 


SYSTEMS  OP  SATELLITES. 

THE  SATELLITES  OF  SATURN: 

Mimas. •••        117,000  0       22       37         5 


P^nceladus       150,000 

Tethys 186,000 

Dione 238,000 

Rhea 332,000 

Titan 771,000 

Hyperion        934,000 

lapetus            2,225,000 


1  8  53  7 

1  21  18  26 

2  17  41  10 
4  12  25  12 

15  22  41  27 

21  6  38  24 

79  7  56  23 


p.  „,  In  ' ^UJ>>  190f-  Pro£  E-  C.  Pickering  announced  the  confirmation  of  the  discovery  of  a  ninth  satellite  of  Saturn, 
nailer '        '  °n  Phot°SraPhs  m  ls99-    Its  period  is  about  1£  years,  and  its  distance  from  Saturn  about  8,000,000 

THE  SATELLITES  OF  UKANUS  : 

Mean  distance  from  Periodic  Time. 

Centre  of  Planet.  Days      hrs.      mins.     sees. 

Ariel 120,000  miles  2  12  29  21 

Umbriel          167,000      „  4  3  27  37 

Titania            273,000      „  8  16  56  30 

Oberon            365,000      „  13  11  7  6 

THI:  SATELLITK  OF  NEPTUNE  : 

(Nameless)      221,500      „  5       21         2      38 

The  satellites  of  Mars  are  a  remarkable  pair.  The  inner,  Phobos,  is  the  only  satellite 
that  revolves  faster  than  its  primary  rotates.  In  consequence  of  this,  it  must  rise  in  the 
west,  and  set  in  the  east.  The  outer  satellite,  Deimos,  revolves  in  a  period  so  little  greater 
than  that  of  the  planet  that  it  goes  through  all  its  phases  twice  between  the  times  of  rising 
and  setting. 

The  four  well  known  satellites  of  Jupiter  are  almost  always  called  by  their  numbers ; 
their  names,  which  have  almost  fallen  into  disuse,  are  therefore  placed  in  brackets.  The  fifth 
satellite,  discovered  by  Barnard  in  1892,  still  remains  without  a  name. 

Recent  measures  have  given  the  following  values  for  the  diameter  of  the  four  large 
satellites  of  Jupiter,  and  of  Titan,  the  largest  of  the  satellites  of  Saturn. 

I.  2,500  miles.  III.  3,600  miles. 

II.  2,200     „  IV.    3,300     „ 

Titan        ...        2,900  miles. 

But  it  is  very  probable  that,  on  account  of  the  effect  of  irradiation,  these  diameters  may 
be  several  hundred  miles  too  large. 

The  diameters  of  the  other  satellites  of  our  system  are,  for  the  present,  beyond  the  reach 
of  measurement.  If  we  measure  the  amount  of  light  they  reflect  from  the  sun,  and  make 
some  assumption  as  to  their  albedo,  or  light  reflecting  power,  we  can  estimate  roughly  the 
probable  diameters  of  the  others.  One  finds  that  the  Phobos  and  Deimos  are  probably  about 
10  and  30  miles  in  diameter,  the  fifth  satellite  of  Jupiter,  100. 


PLATE   8. 
MAP  OF  MARS. 

The  selection  for  this  work  of  a  representation  of  the  surface  of  the  planet  Mars  is  a 
matter  of  great  difficulty.  Observers  of  Mars  are  divided  into  two  camps— those  who  see  the 
canals  and  those  who  do  not.  The  former  are  in  the  strong  position  that  they  are  perfectly 


12  POPULAR   GUIDE   TO   THE   HEAVENS. 

sure  that  they  see  what  they  represent  in  their  drawings  ;  the  latter  declare  that  under  the 
finest  possible  conditions  of  observation,  and  with  the  most  perfect  instruments,  they  can  see 
nothing  resembling  the  straight  markings  which  are  known  as  canals.  And  further,  they 
bring  forward  experiments  which  make  it  clear  that  irregularly  disposed  markings  imperfectly 
seen,  give  the  effect  of  straight  streaks,  by  an  optical  illusion.  The  interest  which  has  been 
excited  by  the  speculations  based  upon  the  drawings  of  these  apparently  artificial  markings 
makes  it  impossible  to  present  a  chart  of  Mars  in  which  the  canals  are  omitted.  We  give, 
therefore,  a  reproduction  of  the  chart  of  Mars  made  at  the  Lowell  Observatory,  Flagstaff, 
Arizona,  by  Mr.  A.  E.  Douglass,  from  a  study  of  all  the  drawings  made  there  by  various 
observers  during  the  opposition  of  1896-97.  At  the  same  time  it  is  necessary  to  give  the 
caution  that  some  of  the  very  best  observers  deny  altogether  the  truth  of  this  representation 
of  the  planet. 

Our  difficulty  is  increased  by  the  fact  that  there  are  two  rival  systems  of  nomenclature  for 
the  features  of  Mars — an  earlier  system  in  which  the  so-called  lands  and  seas  are  named 
after  modern  Astronomers — Herschel,  Leverrier,  Dawes,  &c.,  and  a  later,  in  which  the  names 
are  taken  from  classical  geography  and  mythology.  The  later  system  seems  likely  to 
prevail,  and  we  have  adopted  it  in  the  present  work.  It  is  useless  to  give  a  catalogue  of  some 
400  names  of  markings  whose  very  existence  is  in  dispute.  We  confine  ourselves  therefore  to 
naming  some  of  the  more  prominent  features,  to  which  a  number  is  affixed  in  the  plate. 

1.  Fastigium  Aryn.  13.  Mare  Tyrrhenium. 

2.  Margaritifer  Sinus.  14.  Syrtis  Minor. 

3.  Mare  Erythraeum.  15.  Syrtis  Major. 

4.  Aurorae  Sinus.  16.  Cerberus  (Canal). 

5.  Ganges  (Canal).  17.  Marc  Icarium. 

6.  Lunae  lacus.  18.  Edom  promontorium. 

7.  Solis  lacus.  19.  Hellas. 

8.  Sirenius  lacus.  20.  Ausonia. 

9.  Mare  Sirenum.  21.  Trivium  Charontis 

10.  Eumenides  (Canal).  22.  Orcus  (Canal). 

11.  Mare  Cimmerium.  23.  Pyriphlegethon. 

12.  Charontis  lacus.  24.  Mare  Chronium. 

It  should  be  understood  that  in  the  unsteady  air  of  England  it  is  almost  hopeless  to 
expect  to  see  many  of  the  finer  details.  Not  even  in  the  most  favourable  climates  are  they 
to  be  seen  always,  or  all  at  once.  And  much  training  of  the  eye  is  required  before  it  is  fit  to 
decide  for  or  against  the  existence  of  these  details  on  the  very  verge  of  invisibility. 


PLATU    9. 
JUPITER  AND  SATELLITE  I. 

Owing  to  the  absence  of  permanent  features  on  Jupiter  it  is  not  possible  to  give  a  map 
of  the  planet.  From  year  to  year  the  position  and  breadth  of  the  belts  change,  the  tints  of 
the  surface  change,  and  the  shape  and  character  of  the  spots  change.  Under  these  circum- 
stances the  best  that  can  be  done  is  to  present  drawingsof  the  planet  which  are  typical,  yet  possess 
features  of  more  than  average  interest.  We  therefore  select  a  set  of  drawings  covering  the 
period  when  the  ''  great  red  spot "  was  most  conspicuous.  It  was  first  seen  in  July  1878,  and 


JUPITER.  13 

in  the  following  year  it  was  the  most  conspicuous  feature  on  the  planet  (Figs.  4,  5,  6).  In 
1880  and  1881  it  changed  but  little  (Figs.  7,  8,  9, 12),  but  after  that  began,  to  fade  ;  and  at  the 
present  time  it  is  visible  only  as  an  indentation  or  scar  on  the  southern  equatorial  belt.  It 
was  evident  almost  from  the  first  that  its  period  of  rotation  was  not  the  same  as  that  of  the 
average  spot  in  the  belt  near  it.  These  gained  22  sees,  upon  it  at  each  rotation.  And  though 
the  spot  is  more  or  less  permanent  its  own  time  of  rotation  has  changed  by  6s,  and  for  these 
facts  no  satisfactory  theory  has  been  suggested. 

The  satellites  of  Jupiter  were  the  first  discoveries  made  with  the  telescope,  and  they 
remain  the  most  beautiful  and  interesting  objects  that  a  small  telescope  can  show.  Their 
eclipses  and  occupations  and  transits  over  the  planet's  disc  are  predicted  in  the  Nautical 
Almanac  year  by  year,  to  which  reference  may  be  made  also  for  the  configuration  of  the 
satellites  each  night. 

With  a  very  powerful  telescope  the  phenomenon  of  the  transit  of  satellite  I.  is  very 
curiously  varied.  The  figures  are  from  drawings  made  by  Prof.  Barnard  at  the  Lick  Observa- 
tory. It  had  been  noticed  that  when  Satellite  I.  was  crossing  the  disc  of  the  planet,  is 
sometimes  appeared  double  and  sometimes  very  elongated.  The  drawings  supply  the 
explanation.  The  satellite,  not  unlike  its  primary,  has  a  bright  equatorial  region  and  darker 
poles.  When  it  is  projected  upon  a  dark  belt  of  a  planet  the  former  alone  is  seen  ;  when  upon 
a  bright  belt  the  latter.  The  drawing  made  November  19,  1893,  shows  the  phenomena  most 
completely.  The  satellite  was  seen  against  the  boundary  separating  a  bright  from  a  dark 
belt ;  and  it  was  also  partly  superposed  upon  its  own  shadow.  It  is  scarcely  necessary  to  add 
that,  since  the  whole  apparent  diameter  of  the  satellite  is  little  more  than  a  second  of  arc,  it 
requires  the  finest  telescope  and  skill  to  see  what  is  here  shown. 


PLATE  10. 
SATURN. 

We  are  indebted  again  to  Professor  Barnard  and  the  Lick  telescope  for  the  drawing  which 
has  been  chosen  to  illustrate  the  appearance  of  the  planet  Saturn.  Although  spots  are 
sometimes  seen  upon  the  planet,  they  are  uncommon,  and  the  surface  markings  are  usually 
no  more  than  a  few  vague  dusky  belts  ;  the  interest  lies  in  the  rings. 

In  looking  at  the  plate  we  must  imagine  the  sun  behind  us  and  a  little  to  the  left.  The 
shadow  of  the  ball  is  seen  upon  the  rings  (at  the  right  hand  limb)  and  the  shadow  of  the  rings 
is  seen  upon  the  ball  (above).  The  Cassini  division  was  plainly  visible  all  round,  but  the 
Encke  division  in  the  outer  ring  was  not  visible  at  the  time  ;  it  seems  to  be  a  thin  place  in 
the  ring  rather  than  an  actual  division.  The  dusky,  or  crape  ring,  showed  steely  blue  against 
the  sky,  and  at  its  inner  edge  was  so  transparent  that  the  planet  could  be  seen  through  it. 
Where  it  joins  the  inner  ring  there  is  no  division,  but  the  two  rings  merge  rapidly  the  one  into 
the  other.  The  brightest  part  of  the  whole  is  the  outer  edge  of  the  inner  bright  ring. 

Since  this  drawing  was  made  the  rings  have  opened  out  to  their  fullest  extent,  and  are 
now  (1903)  closing  in  again  as  the  planet  approaches  the  interesting  point  in  its  orbit  where 
the  rings  are  seen  edgewise. 


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CHAPTER     III. 

PLATES    11    &    12. 
THE  SUN. 

A  very  important  branch  of  the  work  of  the  Royal  Observatory,  Greenwich,  is  the  daily 
record,  by  photography,  of  the  number  and  size  of  the  spots  which  appear  upon  the  Sun's 
surface.  To  fill  the  gaps  caused  by  cloudy  weather  in  England  photographs  are  taken  also 
in  India  and  Mauritius,  and  are  sent  home  to  Greenwich,  so  that  there  are  very  few  days  in 
the  year  for  which  there  is  no  record.  The  purpose  of  this  continuous  survey  of  the  Sun  is 
to  determine  the  laws  which  govern  the  changes  in  the  area  and  position  of  the  spots.  It  is 
well-known  that  the  number  of  spots  reaches  a  maximum  about  every  eleven  years  ;  that  at 
the  beginning  of  each  new  period  the  spots  are  found  in  higher  solar  latitudes  than  at  the  end  ; 
and  that  there  is  an  unmistakable,  but  unexplained,  connection  between  the  frequency  of  Sun 
spots,  of  displays  of  the  Aurora  Borealis,  and  of  terrestrial  magnetic  storms.  One  of  the 
finest  Sun-spot  photographs  ever  taken  at  Greenwich  is  reproduced,  by  permission  of  the 
Astronomer  Royal,  in  Plate  11.  The  structure  of  the  group  is  very  complex.  Every  large 
spot  is  accompanied  by  a  crowd  of  smaller  spots,  which  change  comparatively  quickly.  A 
large  regular  spot  consists  of  two  well-defined  portions — the  black  central  -umbra  and  the 
surrounding  grey  penumbra.  In  the  latter,  the  bright  granules  which  form  the  photosphere 
of  the  Sun  are  elongated  and  drawn  in  towards  the  centre  of  the  spot,  making  the  structure 
of  the  photosphere  somewhat  like  thatch.  Very  frequently  bright  bridges  are  thrown  across 
from  one  side  to  the  other,  and  this  is  generally  the  prelude  to  the  filling  up  of  the  spot. 

Sun  spots  are  the  seat  of  tremendous  activity  in  the  layers  of  glowing  gas  lying  above  the 
photosphere.  The  most  remarkable  of  the  gaseous  prominences,  which  stand  out  above  the 
limb  of  the  Sun  when  it  is  totally  eclipsed,  are  almost  always  associated  with  spots  lying 
beneath  them.  The  lower  part  of  Plate  12  is  the  reproduction  of  a  photograph  taken  by 
Prof.  Barnard  and  Mr.  Ritchey  during  the  total  eclipse  of  1900 — May  28th.  These 
prominences  are  outbursts  of  hydrogen,  calcium,  and  occasionally  of  other  metallic  vapours, 
which  are  often  thrown  up  from  the  surface  of  the  Sun  with  enormous  velocities.  The 
prominences  are  conspicuously  seen  during  a  total  eclipse  because  the  glare  in  our  atmosphere, 
which  ordinarily  surrounds  the  Sun,  is  then  for  the  moment  removed.  The  application  of  a 
spectroscopic  method  now  enables  us  to  abolish  the  effect  of  this  glare  at  any  time,  and  it  is 
possible  to  make  a  daily  record  of  the  prominences. 

The  upper  part  of  Plate  12  is  a  reproduction  of  a  set  of  drawings  of  a  single  prominence 
made  in  this  manner  by  Herr  Fenyi,  at  Kalocsa  in  Hungary,  on  1895,  July  15.  His  des- 
cription is  as  follows.  At  7.10  a.m.,  Greenwich  M.T.,  a  very  delicately  formed  prominence 
stood  precisely  on  the  place  where  a  considerable  group  of  Sun  spots  was  passing  out  of  sight 
round  the  limb  (Fig.  1).  When  Fig.  2  was  drawn  at  7.40,  the  form  of  the  prominence  was 


16  POPULAR   GUIDE   TO   THK   HEAVENS. 

changing  with  extraordinary  rapidity.  Determinations  of  the  velocity  with  which  parts  of 
the  prominence  were  moving  gave  results  up  to  500  miles  per  second.  Fig.  3  was  drawn  at 
8.7 ;  Fig.  4  at  8.30,  when  the  prominence  had  reached  its  greatest  height  of  about  100,000 
miles.  At  8.45  its  shape  had  changed  very  much,  and  at  9.35,  when  Fig.  6  was  drawn,  the 
great  protuberances  had  completely  gone,  and  the  prominence  had  returned  to  nearly  its 
appearance  of  2i  hours  before. 


PLATE  13. 
PATHS  OF  SPOTS  ACROSS  THE  SUN'S  DISC. 

By  the  rotation  of  the  Sun  on  its  axis,  the  spots  appear  to  be  carried  across  the  disc, 
along  paths  parallel  to  its  equator. 

The  axis,  around  which  the  Sun  rotates,  is  inclined  to  the  ecliptic  at  an  angle  of  82°  45'. 
The  inclination  of  the  Sun's  equatorial  plane  to  the  ecliptic  is  therefore  7°  15'. 

The  ascending  node  of  the  Sun's  equator  is  the  point  at  which  a  spot  on  the  equator  of  the 
Sun  would  be  carried  by  the  Sun's  rotation  from  the  southern  to  the  northern  side  of  the 
ecliptic,  and  the  longitude  of  the  node  is  the  angle  which  the  direction  of  this  point  makes 
with  the  direction  of  the  First  Point  of  Aries  as  seen  from  the  Sun's  centre.  The  actual 
value  of  the  longitude  of  the  ascending  node  is  74°.  Its  position  is  marked  on  Plate  2. 

Plate  13  shows  the  paths  along  which  the  spots  appear  to  travel  at  different  dates.  They 
are  here  represented  as  actually  on  the  face  of  the  Sun,  and  not  as  seen  through  the  inverting 
telescope  that  the  astronomer  ordinarily  uses. 

On  December  6th,  the  Earth  is  in  the  line  of  nodes,  and,  consequently,  in  the  plane  of  the 
Sun's  equator,  and  the  paths  pursued  by  the  spots  will  therefore  appear  projected  into  straight 
lines.  Again,  on  June  5th,  when  the  Earth  is  in  the  opposite  point  of  its  orbit,  it  will  be 
again  in  the  plane  of  the  Sun's  equator,  and  the  paths  of  the  spots  will  again  appear  projected 
into  straight  lines. 

On  March  4th,  the  Earth,  being  then  90°  from  the  node,  will  be  depressed  below  the  Sun's 
equator  by  an  angle  of  7Q  15',  and  the  paths  of  the  spots  will  appear  as  ellipses  of  considerable 
curvature,  with  their  convexities  towards  the  north  ;  while,  on  September  6th,  from  the  op- 
posite point  of  the  orbit,  the  same  curves  will  reappear,  only  that  they  will  now  be  convex 
towards  the  south.  From  March  till  June,  and  from  September  till  December,  the  curvature 
is  decreasing,  while  in  the  intervening  periods  corresponding  changes  take  place  in  the  opposite 
direction.  We  may  describe  these  changes  in  a  somewhat  different  way  by  saying  that  on 
June  5th  and  December  6th  both  poles  of  the  Sun  are  visible  just  on  the  edge  of  its  disc  ; 
from  June  to  December  the  north  pole  only  is  visible  j  and  from  December  to  June  the  south 
pole  only  can  be  seen. 

By  the  "  position  angle  of  the  Sun's  axis,"  is  meant  the  angle  which  the  projection  of  the 
northern  half  of  the  Sun's  axis  on  its  apparent  disc  makes  with  the  meridian  passing  through 
the  Sun's  centre,  reckoned  positive  towards  the  eastern,  and  negative  towards  the  western 
side  of  the  disc.  If  the  observation  is  made  at  noon,  it  is  the  angle  which  the  direction  of  the 
axis  makes  with  the  vertical,  when  the  image  is  viewed  projected  on  a  sheet  of  paper  placed 
behind  the  eyepiece  of  an  inverting  telescope.  If  the  observer's  back  be  turned  towards  the 


THE    SUN.  17 

Sun,  the  position  angle  will  be  positive  when  the  upper  half  of  the  axis  leans  towards  the 
right,  and  negative  when  it  leans  towards  the  left.  On  such  a  projection  the  cardinal  points, 
N.,  S.,  E.,  W.,  lie  just  as  they  do  in  an  ordinary  terrestrial  atlas.  On  January  5th  and 
July  6th,  the  position  angle  of  the  Sun's  axis  is  Zero  ;  from  July  6th  it  gradually  increases  in 
a  positive  direction  until  it  reaches  its  greatest  value,  viz.  :  +  26°  20',  on  October  10th.  From 
this  date  it  gradually  diminishes  till  January  5th,  alter  which  it  becomes  negative,  reaching 
its  greatest  negative  value,  viz. :  -  26°  20',  on  April  5th,  and  returning  once  more  to  Zero  on 
July  6th. 

PLATE  14. 
PHASES  OF  THE  MOON:  LUNAR  AND  SOLAR  ECLIPSES. 

Phases  of  the  Moon. — The  hemisphere  of  the  Moon  that  is  turned  towards  the  Sun  is, 
of  course,  brilliantly  lighted ;  the  other  hemisphere  is  dark.  As  the  Moon  moves  in  its 
orbit  round  the  Earth,  the  illuminated  side  is  turned  towards  us  in  varying  proportions;  and 
the  relation  between  the  phases  thus  produced  and  the  relative  positions  of  Sun,  Earth,  and 
Moon  is  shown  in  the  upper  part  of  the  plate. 

When  the  Moon  is  but  a  few  days  old,  and  appears  as  a  thin  crescent,  it  frequently 
happens  that  part  of  the  Moon  which  is  not  lit  directly  by  the  Sun  is  seen  faintly 
shining  by  the  reflected  "  Earth-light,"  an  appearance  known  as  "  the  old  Moon  in  the  new 
Moon's  arms." 

Eclipses  of  the  Moon. — The  Sim  throws  behind  the  Earth  a  dark  cone  of  shadow, 
which  reaches  a  long  way  beyond  the  path  of  the  Moon,  and  if  it  had  happened  that  the  path 
of  the  Moon  lay  precisely  in  the  ecliptic,  then,  at  every  full  Moon,  she  would  pass  through 
this  shadow,  and  be  totally  eclipsed.  Since,  however,  the  Moon's  path  makes  a  small  angle 
with  the  ecliptic,  she  usually  passes  a  little  above  or  below  the  cone  of  shadow,  and  escapes 
eclipse.  But,  from  time  to  time,  the  Moon  is  crossing  the  ecliptic  just  about  the  time  of  full, 
and  then  a  partial  or  total  eclipse  occurs,  and  is  visible  over  the  whole  of  that  hemisphere  of 
the  Earth  which  is  at  the  moment  turned  away  from  the  Sun  and  towards  the  Moon.  It 
lollows  that  a  total  eclipse  of  the  Moon— being  visible,  whenever  it  occurs,  over  at  least  half 
the  Earth — is  not  a  very  uncommon  spectacle. 

Eclipses  of  the  Sun. — The  Sun  also  throws  behind  the  Moon  a  dark  cone  of  shadow  ; 
smaller  than  that  thrown  behind  the  Earth,  because  the  Moon  is  smaller ;  but  just  long 
enough,  on  the  average,  to  reach  the  Earth.  When  the  Moon  is  nearest  the  Earth,  the  cone 
of  shadow  may  cover  a  space  about  170  miles  broad  ;  and,  with  the  motion  of  the  Moon,  this 
shanow-patch  sweeps  quickly  over  the  Earth.  Within  the  shadow-belt,  for  a  few  minutes, 
the  Sun  is  just  a  little  more  than  completely  obscured  by  the  Moon,  and  there  is  a  total 
eclipse  of  the  Sun.  When  the  Moon  is  iarthest  from  the'  Earth,  the  shadow-cone  does  not 
reach  the  Earth,  so  that  from  no  point  can  the  Sun  be  seen  completely  obscured  ;  at  best, 
there  is  a  ring  of  Sun  showing  all  round  the  Moon,  and  the  eclipse  is  annular,  At  points 
lying  outside  the  belt  of  totality,  or  of  annularity,  the  Sun  may  be  partially  obscured  by  the 
Moon,  and  there  is  a  partial  eclipse.  But  the  limits  within  which  any  eclipse  at  all  is 
visible  are  far  within  the  boundary  of  the  whole  hemisphere  of  the  Earth  which  is  turned 
towards  the  Sun,  and  consequently  eclipses  of  the  Sun  of  any  kind  are  much  more  rarely 
seen  at  any  one  place  than  are  eclipses  of  the  Moon. 


18  POPULAR  GUIDE  TO  THE  HEAVENS. 

PLATE  15. 
PATHS    OF    TOTAL    ECLIPSES    OF    THE    SUN,    1901-1950. 

In  his  great  work,  "Canon  der  Finsternisse,"  Prof.  Oppolzer  has  given  maps  of  the  paths 
of  the  Moon's  shadow  over  the  surface  of  the  Earth  for  all  the  total  and  annular  eclipses  of 
the  Sun  between  the  years  1207  B.C.  and  2162  A.D.  From  this  work  Plate  15  has  been 
prepared,  showing  the  tracks  of  the  total  eclipses  visible  between  1901  and  1950  A.D.  At 
the  western  end  of  each  line  the  eclipse  begins  at  sunrise  ;  the  point  in  the  middle  of  each 
line,  where  the  eclipse  is  at  noon,  is  marked  by  a  circle  ;  and  at  the  eastern  end  of  the  line 
the  eclipse  begins  at  sunset. 

An  examination  of  these  curves  will  show  in  a  striking  way  the  repetition  of  eclipses  in 
a  period  of  about  eighteen  years  and  eleven  days,  which  period,  known  to  the  Chaldeans,  is 
called  the  Saros.  Take,  as  an  example,  the  Great  Eclipse  of  1901— May  18th,  occuring  at 
inid-day  in  long.  97°  E.,  lat.  2°  5'.  We  have  on  our  Plate  three  eclipses  of  this  series,  viz. : — 

Lat.  Long. 

1901— May  18         ...        Eclipse  at  mid-day  in  97°  E.          2°  S. 
1919—    „     29          ...  „  „  18    W.         4    N. 

1937— June  8          ...  „  „          131    W.       10    N. 

And  later  eclipses  of  the  same  series  are  : — 

1955— June  20        ...        Eclipse  at  mid-day  in  117°  E.  15°  N. 

1973—    „    30        ...  „  „  6    W.  19    N. 

1991— July  11         ...  „  „  105    W.  22    N. 

The  centre  of  the  track  of  the  eclipse  is  gradually  moving  north,  and  is  at  each  repetition 
about  seven  hours,  or  105°,  farther  west  in  longitude. 

Let  us  take  as  another  example  the  history  of  the  eclipse  which  will  be  total  in  England 
in  June,  1927.  The  dates  of  the  three  eclipses  of  this  series  represented  on  our  Plate  are  : — 
1909— June  17.  |  1927— June  29.  |  1945— July  9. 

The  first  begins  in  Siberia,  crosses  close  to  the  North  Pole,  and  runs  down  the  west 
coast  of  Greenland.  The  second  begins  in  the  Atlantic,  south-west  of  Ireland,  crosses  Great 
Britain,  runs  up  Norway,  through  the  Arctic  Ocean,  and  ends  south  of  Behring  Straits.  The 
third  begins  in  Canada,  crosses  Greenland  and  northern  Norway,  and  ends  in  Central  Asia. 

It  will  be  seen  that  the  circumstances  of  the  path  of  an  eclipse  are  very  complex, 
especially  when  its  centre  is  in  high  latitudes,  and  the  reason  for  this  may  be  readily  under- 
stood if  one  looks  at  a  globe,  tilted  with  respect  to  the  Sun,  according  to  the  time  of  the  year 
of  the  eclipse,  and  considers  how  the  shadow  of  a  body,  the  Moon,  passing  between  the  Sun 
and  the  globe  would  cut  across  the  tilted  lines  of  latitude  and  longitude. 


PLATE  16. 
TYPICAL    SOLAR    CORONAE. 

By  far  the  most  beautiful  feature  of  the  totally-eclipsed  Sun  is  the  corona  of  pale  white 
light  which  Hashes  out  as  soon  as  the  dazzling  photosphere  is  completely  covered  by  the 


TYPICAL  SOLAR  COR01OE.  19 

Moon.  In  the  three  or  four  minutes  which  is  the  average  duration  of  totality  it  is  almost 
impossible  to  draw  or  describe  the  very  complex  structure  of  this  appendage  of  the  Sun. 
But,  for  the  last  twenty-five  or  thirty  years,  almost  every  eclipse  has  been  successfully 
photographed.  A  reference  to  Plate  15  will  show  the  arduous  character  of  the  journeys 
which  are  often  involved  in  eclipse-observation.  The  six  photographs  which  are  reproduced 
in  Plate  16  were  taken  as  follows  : 

1.  1871.    Dec.  12.      H.  Davis.  Baikul,  India. 

2.  1882.     May  17.      Abney  aud  Schuster.        Egypt. 

3.  1893.     April  16.     J.  Kearney.  Fundium,  W.  Africa. 

4.  1878.     July  29.      W.  Harkness.  Wyoming. 

5.  1889.    Jan.  1.        W.  H.  Pickering.  California. 

6.  1900.     May  28.      E.  E.  Barnard.  Wadesborough,  N.  Carolina. 

The  photographs  have  been  arranged  in  two  sets,  in  which  it  will  be  seen  that  the  corona 
is  of  distinctly  different  types.  In  the  first  set — 1871, 1882, 1893— the  corona  is  fairly  equally 
distributed  right  round  the  limb  of  the  Sun  ;  in  the  second  set— 1878,  1889,  1900 — the 
corona  has  large  equatorial  extensions,  aud  at  the  poles  it  is  broken  up  into  short,  distinct 
streamers.  Further,  it  will  be  noticed  that  the  interval  between  successive  photographs  in 
each  set  is  about  eleven  years  — the  sun-spot  period;  the  first  set  fall  near  the  times  of 
sun-spot  maximum,  the  second  near  times  of  sun-spot  minimum. 

Not  very  much  is  known  of  the  nature  of  the  corona.  The  streamers  shine  largely,  if 
not  entirely,  by  reflected  light  from  the  Sun,  and  must,  therefore,  be  composed  of  small 
particles.  Diffused  amongst  them,  but  probably  not  sharing  in  their  radial  structure,  is  an 
unknown  gas — called,  for  convenience,  "  coronium."  That  some  of  the  detailed  structure  of 
the  corona  is  connected  with  underlying  sun-spots  and  prominences  is  certain.  But  the  most 
significant  fact  is  the  evident  dependence  of  the  forces  which  determine  the  form  of  the 
corona  upon  the  same  cause,  whatever  it  may  be,  which  produces  the  periodicity  of  the  sun- 
spots,  disturbance  of  the  magnet,  and  aurorae. 

The  corona  and  prominences  alike  are  ordinarily  invisible  to  us,  because  they  are  not 
nearly  so  bright  as  the  flare  in  our  atmosphere  which  seems  to  surround  the  Sun.  The 
spectroscope  has  made  it  possible  to  observe  the  prominences  continuously ;  but,  up  to  the 
present,  no  method  has  been  found  of  viewing  the  corona  except  during  the  rare  minutes  of  a 
total  eclipse. 


Plate  12. 


BALL'S  POPULAR  GUIDE  TO  THE    HEAVENS. 

SOLAR  PROMINENCES.  Drawn  with  the 
spectroscope  by  J.  FENYI,  1895,  July  15th. 


SOLAR    PROMINENCES.  PhotoHraohed  by  E.  E.  BARNARD  durina  the  Total  Eclipse  of  the  Sun.  1900,  May  28th. 


>x\>^i:' 

x^ 

'UNIVERSITY 

of 


BALL'S   POPULAR   GUIDE  TO  THE    HEAVENS. 


Plate   16. 


(21) 


CHAPTER   IV. 

PLATE  17. 
DONATI'S    COMET. 

This,  the  most  famous  comet  of  the  19th  Century,  was  discovered  by  Donati  at  Florence, 
on  June  2nd,  1858,  as  a  small  telescopic  object  approaching  the  Sun.  Not  for  nearly  three 
months  did  it  become  visible  to  the  naked  eye,  but  thence,  right  up  to  the  time  of  its 
perihelion  passage,  at  the  end  of  September,  it  grew  rapidly  in  brightness  until  its  starlike 
nucleus  was  as  bright  as  the  Pole  star.  During  September  its  tail  was  directed  nearly 
towards  the  Earth,  and,  though  bright,  was  seen  so  much  foreshortened  that  its  effect  was 
greatly  marred ;  but  as  the  comet  passed  perihelion  and  began  to  recede  from  the  Sun,  its 
path,  by  good  fortune,  was  most  favourably  placed.  The  splendid  plumed  tail  then  lay  almost 
at  right  angles  to  the  line  of  sight,  and  its  whole  length  was  for  the  first  time  displayed. 
Other  comets  have  had  longer  tails,  though  this  was  more  than  forty  million  miles  long,  but 
none  have  surpassed  Donati's  comet  in  beauty.  The  main  tail,  the  curved  plume,  was  of  the 
type  shown  afterwards  by  the  spectroscope  to  consist  of  hydrocarbons ;  the  thin  straight 
streamers  are  of  the  hydrogen  type.  Evaporated,  apparently,  from  the  nucleus  of  the  comet 
by  the  heat  of  the  Sun,  the  particles  of  the  tail  are  repelled  from  the  Sun  by  some  force 
whose  nature  is  still  problematical,  and  driven  backwards  from  it  with  a  speed  which  must  be 
comparable  with  that  of  the  speed  of  light  itself. 

On  the  evening  of  October  5th,  Donati's  comet  was  at  its  best,  when  its  motion  involved 
the  bright  star  Arcturus  in  the  brightest  part  of  its  tail,  through  which  the  star  shone 
imdimmed.  Our  plate,  which  was  drawn  by  Prof.  Bond,  at  the  Harvard  College  Observatory, 
shows  Arcturus  close  to  the  comet's  head,  while  its  tail  sweeps  up  between  the  Great  Bear 
and  the  Northern  Crown. 


PLATE  18. 

No.  1. 
HOLMES'  COMET  AND  THE  ANDROMEDA  NEBULA. 

On  Nov.  6,  1892,  Mr.  Edwin  Holmes  discovered  in  London  a  comet  which  was  in  many 
ways  remarkable.  When  found  it  was  close  to  the  great  nebula  in  Andromeda,  and  its 
motion  was  so  slow  that,  throughout  the  month  of  November,  it  could  be  photographed  on 
the  same  plate  with  the  nebula.  Plate  18  is  a  reproduction  of  a  photograph  taken  at  the 
Lick  Observatory,  on  November  10th,  by  Professor  Barnard,  who  describes  the  comet  as 


22  POPULAR  GUIDE   TO,  THE   HEAVENS. 

"round,  and  sharply  defined  like  a  planetary  nebula,  with  a  symmetrical,  nebulous 
atmosphere  surrounding  it  for  some  distance." 

The  after-history  of  this  comet  is  very  curious.  By  the  middle  of  December,  it  had 
grown  so  exceedingly  faint  and  ill-defined  that  scarcely  any  telescope  could  show  it.  But,  in 
the  middle  of  January,  it  suddenly  brightened  up,  and  condensed  into  a  small,  hazy,  star-like 
object,  after  which  it  again  became  diffuse,  and  finally  vanished. 

The  comet's  orbit  was  equally  remarkable.  It  lay  entirely  between  Mars  and  Jupiter,  in 
the  zone  of  the  minor  planets  ;  and  it  has  even  been  suggested  that  the  comet  was  not  a 
comet  at  all,  but  the  result  of  some  celestial  accident — such  as  a  collision — which  had  befallen 
an  asteroid. 


Nos.  2  AND  3. 
COMET  a  1893,  IV.     (BROOKS.) 

» 

This  comet,  though  small — and,  as  a  visual  object,  insignificant — was,  in  some  ways,  the 
most  remarkable  comet  that  has  yet  been  studied  by  photography.  The  plate  is  a  reproduc- 
tion of  part  of  a  series  of  photographs  taken  by  Professor  Barnard  at  the  Lick  Observatory. 
The  motion  of  the  comet  was  towards  the  north-east,  the  left-hand  top  corner  of  the  picture. 
On  1893,  Oct.  20th,  the  tail  was  straight,  but  gradually  widening  towards  the  end  ;  on  the 
next  day,  the  date  of  the  second  picture,  it  had  been  completely  transformed.  The  tail  is  very 
much  distorted,  as  if  the  matter  of  which  it  is  formed  had  encountered  some  resistance.  On 
the  following  day,  October  22nd,  the  tail  was  completely  wrecked,  and  large  portions  of  it 
were  detached.  In  our  ignorance  of  the  way  in  which  a  comet's  tail  is  produced  and  main- 
tained, it  is  scarcely  possible  to  say  anything  definite  by  way  of  explanation  of  these  changes. 
That  the  comet  had  encountered  some  resisting  medium  is  a  plausible  conjecture,  but  nothing 
more. 


PLATE  19. 
COMET    1901.     I. 

The  Great  Comet  of  1901,  visible  in  the  Southern  Hemisphere,  was  by  far  the  finest 
comet  that  had  been  seen  for  twenty  years.  It  appeared  very  suddenly  on  April  24th,  and 
was  discovered  independently  by  several  persons  in  South  Africa  and  Australia.  It  was  then 
at  perihelion,  and  visible  only  just  before  sunrise,  but  during  the  succeeding  days  it  passed, 
apparently,  still  closer  to  the  Sun,  and  was  lost  in  the  daylight.  By  May  3rd  it  was 
sufficiently  clear  of  the  Sun  to  be  visible  in  the  evening  twilight,  and  on  May  4th  the 
photograph,  from  which  Plate  19  is  made,  was  taken  at  the  Royal  Observatory,  Cape  of  Good 
Hope,  with  the  Victoria  telescope,  in  twilight.  The  tail  is  noticeably  unsymmetrical, 
streaming  from  each  side  of  the  nucleus,  but  much  more  strongly  on  the  south-west  side. 
About  this  time  there  appeared  on  the  same  side  a  long,  straight,  faint  tail,  making  an  angle 
of  about  30°  with  the  axis  of  the  main  tail,  and  as  the  comet  got  away  from  the  Sun  into 
darker  sky,  this  tail  could  be  traced  for  about  25°,  the  extreme  length  of  the  main  tail  being 
about  7°. 


COMETS.  23 

COMET  b  1902.     III.     (PERRINE.) 

This  was  an  excellent  example  of  the  kind  of  comet  which  raises  false  hopes  when  it  is 
reported  in  the  papers  as  "  visible  to  the  naked  eye."  At  its  brightest  it  was  little  more 
conspicuous  than  the  Andromeda  nebula,  witli  which  few  people  are  familiar  as  a  naked-eye 
object.  ;  in  the  telescope,  it  was  an  almost  formless  patch  of  light,  with  a  vague  tail.  The 
photograph — taken  at  the  Royal  Observatory,  Greenwich,  on  Sept.  29,  1902 — shows  the  tail 
strongly  cleft.  Six  divisions  can  be  counted  in  the  original  from  which  the  plate  was  made. 

This  photograph  was  made  with  an  exposure  of  62m.  The  comet  was  in  rapid  motion 
amongst  the  stars,  and  the  telescope  with  which  the  photograph  was  made  was  kept  pointed 
precisely  to  it ;  in  consequence  of  this,  the  stars  appear  as  trails,  and  give  a  precise  idea  of 
the  amount  by  which  the  comet  had  moved  during  the  hour  which  was  needed  to  secure  this 
picture. 


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CHAPTER    V.—THE  MOON. 

PLATES  20,  21  &  22. 

The  three  photographs  here  reproduced  were  taken  at  the  Yerkes  Observatory  with  the 
great  telescope,  temporarily  converted  into  a  photographic  telescope  by  the  device  of  photo- 
graphing through  a  screen  of  yellow  glass  in  contact  with  the  plate. 

Plate  20  shows  the  region  of  the  Mare  Serenitatis  and  the  Apennines.  The  Mare  is 
more  than  400  miles  across,  and  is  singularly  free  from  Craters.  The  appearance  in  the 
photograph  of  the  curious  serpentine  ridge  towards  its  western  border  is  a  good  example  of 
the  importance  of  selecting  the  right  moment  for  studying  any  particular  lunar  object. 
When  the  photograph  was  taken  this  ridge  was  conspicuous  :  had  it  been  taken  a  few  hours 
later  the  ridge  would  have  disappeared.  It  is  really  very  low,  so  that  it  soon  loses  its  shadow, 
and  as  soon  as  that  happens  it  is  no  longer  distinguishable. 

The  bright,  white  spot  Linn£  has  a  long  history.  It  was  drawn  by  old  observers  as  a 
deep  crater.  For  many  years  it  has  been  merely  a  bright  spot,  with  scarcely  any  depression 
at  all.  Opinions  differ  widely  as  to  the  reality  of  any  change  ;  perhaps,  on  the  whole,  the 
evidence  is  in  favour  of  something  having  happened.  But  the  doubt  as  to  the  trustworthiness 
of  the  old  observations  emphasises  the  value  of  photographs  such  as  these,  which  could 
scarcely  give  a  wrong  verdict  on  such  a  point. 

Craters  differ  much  in  their  brightness  ;  Alfraganus  and  Dionysius  have  exceptionally 
brilliant  walls  ;  Julius  Csesar  and  Boscovich  are  very  dark. 

The  boundary  of  Julius  Csesar  towards  Sosigenes  has  a  broken  down  and  denuded 
appearance  ;  the  deep  valley  alongside  it  has  probably  been  formed  by  the  fusion  of  several 
craters,  which  are  frequently  found  three  or  four  in  a  row  close  together. 

The  Apennines  and  the  Caucasus  of  the  Moon  are  mountainous  regions  much  more 
resembling  those  of  the  Earth  than  do  the  lunar  mountains  in  general.  The  peaks  run  up  to 
18,000  and  20,000  feet,  and  the  N.E.  boundary  of  the  Apennines  is  a  very  steep  cliff,  not  well 
shown  in  the  photograph,  which  shows  it  under  a  setting  sun. 

There  is  a  curious  contrast  between  the  craters  Archimedes  and  Aristillus.  The  former, 
though  50  miles  in  diameter,  has  its  crater  floor  only  some  600  feet  below  the  plain  outside. 
Its  walls,  about  5,000  feet  high,  look  broken  and  denuded,  and  the  crater  has  the  appearance 
of  having  been  filled  up  nearly  to  the  brim  by  an  outflow  of  lava.  In  Aristillus,  on  the 
contrary,  the  depth  from  brim  to  floor  is  11,000  feet ;  the  central  peak  and  terraces  are 
preserved,  and  the  plain  all  round  is  covered  as  if  with  the  debris  of  relatively  late  eruptions. 

In  Plate  21  we  have  a  picture  of  the  most  rugged  and  broken  part  of  the  Moon's  surface. 
The  crater  Tycho  at  sunrise,  as  shown  here,  is  relatively  undistinguished,  though  of  such  size 
that  Mont  Blanc  would  stand  on  its  floor,  and  from  its  summit  it  would  not  be  possible  to  see 
over  the  crater  wall.  But  as  the  Moon  gets  toward  full,  while  most  of  the  other  craters 
become  hard  to  see — Clavius,  for  example,  almost  entirely  disappears  under  the  perpendicular 
illumination — Tycho  stands  out  conspicuously  brilliant,  the  centre  of  a  system  of  radiating 
bright  streaks,  whose  nature  is  a  mystery.  They  go  straight  across  mountains  and  plains  ; 
there  is  only  one  well-marked  case,  Saussure,  in  which  the  streak  seems  to  turn  aside  to  avoid 
a  mountain. 


26  POPULAR   GUIDE   TO   THE   HEAVENS. 

It  is  curious  that  in  the  district  around  Clavius  the  western  walls  of  the  craters  are 
generally  higher  than  the  eastern.  In  Clavius  itself,  a  peak  of  the  western  wall  stands 
17,000  feet  above  the  floor,  and  the  deepest  of  the  smaller  craters  within  is  6,000  feet  deep. 

At  the  extreme  bottom  of  the  picture,  below  and  to  the  left  of  Pitatus,  is  the  Straight 
Wall,  recognised  only  by  its  shadow.  The  wall  is  almost  perfectly  straight,  60  miles  long, 
and  about  1,000  feet  high.  It  is  much  steeper  on  the  east  than  on  the  west  side,  and  is, 
perhaps,  better  called  a  cliff  than  a  wall. 

PLATE  22. — In  Copernicus  and  the  region  around  it  we  find  lunar  scenery  on  the  grandest 
scale.  The  crater  itself  is  about  60  miles  in  diameter  ;  the  highest  peak  is  more  than  12,000 
feet  above  the  floor  ;  the  central  mountain  above  2,000  feet  high.  The  successive  terraces  of 
the  wall  are  said  to  resemble  those  of  the  crater  of  Teneriffe  :  the  ridges  running  down  on  to 
the  plain  suggest  outpourings  of  lava.  To  the  north  is  Mt.  Carpathus  with  an  enormous  c'eft. 
To  the  west  the  whole  plain  is  riddled  like  a  sieve  with  small  craters.  The  line  of  these 
small  craters  running  north  and  south,  and  becoming  at  the  north  end  a  deep  cleft,  suggests 
the  question  :  Are  these  small  craters  formed  along  a  pre-existing  cleft,  or  is  the  cleft,  as  we 
see  it,  formed  by  the  amalgamation  of  a  number  of  small  craters  in  a  line  ? 

PLACE  OF  THE   MOON. 

From  the  monthly  maps,  39 — 50,  the  positions  of  the  Moon  at  different  periods  in  the 
lunation  can  be  learned.  In  the  first  place,  it  is  to  be  noted  that  our  Satellite  lies  always  in 
or  close  to  that  part  of  the  sky  marked  as  the  "  Track  of  the  Planets."  When  it  is  full  the 
Moon  is  in  opposition,  and  comes  on  the  meridian  at  midnight,  and  hence  we  have  the 
following  rule  : 

Look  out  the  monthly  map  for  the  month  in  question,  then  the  full  Moon  lies  in  that 
part  of  the  heavens  where  the  "  Track  of  the  Planets  "  crosses  the  central  meridian,  already 
defined  to  be  the  line  drawn  on  the  map  from  the  North  point  to  the  South  point. 

Example  1. — In  what  Constellation  does  the  full  Moon  appear  in  September  1 

Solution. — The  answer  is  given  by  Plate  47,  where  the  "  Track  of  the  Planets  "  crosses 
the  central  meridian  in  Pisces,  which  indicates  the  required  position. 

Example  2. — When  is  the  full  Moon  near  the  Pleiades  ? 

Solution. — Plate  49  shows  the  Pleiades  on  the  central  meridian,  and  accordingly 
November  is  the  answer  to  the  question. 

To  find  the  position  of  the  Moon  at  the  time  of  the  first  quarter,  the  following  is  the 
method. 

Look  out  the  monthly  map  for  three  months  preceding  the  given  date,  then  the 
constellation  in  or  near  which  the  Moon  lies  at  the  first  quarter  is  shown  at  the  intersection 
of  the  "  Track  of  the  Planets  "  with  the  central  meridian. 

Example. — In  what  constellation  does  the  first  quarter  Moon  appear  in  June  ? 

Solution.— The  map  three  months  earlier  is  Plate  41  for  March.  This  shows  the  inter- 
section of  the  "  Track  of  the  Planets  "  and  the  central  meridian  in  Virgo,  which  is 
accordingly  the  answer  required. 

To  find  the  position  of  the  Moon  at  the  time  of  the  last  quarter,  the  following  is  the  method. 

Look  out  the  monthly  map  for  three  months  following  the  given  date,  then  the 
Constellation  in  or  near  which  the  Moon  lies  at  the  last  quarter  is  shown  at  the  intersection 
of  the  "  Track  of  the  Planets  "  with  the  central  meridian. 


THE   MOON. 


27 


Example. — In  what  constellation  does  the  last  quarter  Moon  appear  in  July  1 

Solution. — The  map  three  months  later  is  Plate  48,  which  shows  that  the  constellation 
is  Aries. 

It  ought  to  be  observed  that,  on  account  of  the  rapid  motion  of  the  Moon,  only  a  rough 
indication  of  its  place  can  be  expected  from  the  process  here  given,  and  that  the  accuracy  will 
be  greater  the  nearer  the  phase  in  question  happens  to  the  middle  of  the  month. 

The  foregoing  problems  can  also  be  solved  by  the  more  general  method  now  to  be 
described.  The  Table  of  Moon  Age  shows  the  position  in  the  heavens  which  the  Moon 
occupies  at  any  age  in  any  month.  The  use  of  this  Table  is  as  follows. 

Enter  the  table  in  the  verticle  column  bearing  the  name  of  the  month.  Then  take  the 
age  in  that  column  nearest  the  given  age,  and  the  figure  at  the  left  on  the  same  row  gives  the 
number  of  the  monthly  map  in  which  the  region  where  the  Moon  is  situated  lies  on  the 
"  central  meridian "  where  the  "  Track  of  the  Planets  "  crosses  it. 

THE  TABLE  OF  MOON  AGE. 


Map. 

Jan. 

Feb. 

March. 

April. 

May. 

June. 

July. 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

39 

14 

12 

10 

7 

5 

3 

29 

25 

23 

20 

18 

16 

40 

17 

14 

12 

10 

7 

5 

3 

27 

25 

23 

21 

19 

41 

19 

16 

15 

12 

10 

8 

5 

2 

28 

25 

23 

21 

42 

22 

19 

17 

14 

12 

10 

8 

5 

1 

0 

26 

24 

43 

25 

22 

21 

16 

14 

12 

10 

7 

4 

2 

28 

26 

44 

27 

25 

23 

18 

16 

14 

11 

9 

7 

4 

2 

29 

45 

29 

27 

25 

20 

18 

16 

14 

11 

9 

7 

5 

2 

46 

2 

0 

27 

25 

21 

18 

16 

14 

11 

9 

7 

4 

47 

5 

2 

29 

27 

24 

20 

18 

16 

14 

11 

10 

7 

48 

7 

4 

3 

0 

27 

23 

20 

18 

16 

14 

12 

9 

49 

10 

7 

5 

3 

0 

27 

23 

20 

18 

16 

14 

11 

50 

12 

9 

8 

5 

2 

0 

27 

23 

20 

18 

16 

14 

Example  1. — Where  does  the  Moon  lie  when  four  days  old  in  October  ? 

Solution. — The  October  column  in  the  Table  of  Moon  Age  being  referred  to,  the 
sixth  figure  from  the  top  gives  4,  the  age  of  the  Moon,  and  the  figure  at  the  end 
of  that  row  on  the  left  is  44.  This  monthly  map  shows  that  the  Moon  must 
then  be  in  or  near  Sagittarius. 

Example  2. — What  will  be  the  age  of  the  Moon  when  on  the  meridian  at  10  P.M.  in 
August  ? 

Solution. — At  10  P.M.  in  August,  the  heavens  will  be  as  in  Plate  45.  Therefore  we 
refer  to  the  row  for  Map  45  in  the  Table  of  Moon  Age,  which  shows,  under  the 
column  August,  that  the  moon  must  then  be  about  11  days  old. 

Example  3. — Determine  when  the  Moon,  at  the  first  quarter,  has  a  specially  high 
altitude. 

Solution. — The  heavens  must  be  as  in  Plate  49,  which  refers  us  to  the  last  row  but 
one  of  the  Table.  For  the  Moon  to  be  7  days  old  we  look  under  the  column 
February,  in  which  month  the  heavens  are  as  in  Plate  49  about  6  P.M. 


POPULAR  GUIDE   TO   THE   HEAVENS. 


PLATES  23  TO  38. 


THE   LUNAR  OBJECTS. 


TERMINOLOGY  OF  LUNAR  QUADRANTS. 


Moon  in  Inverting  Telescope. 


For  the  study  of  the  Lunar  formations,  Plates  23  to  38  have  been  specially  drawn. 

As  the  astronomical  telescope  shows 
the  Moon  turned  upside  down,  and  with 
right  and  left  interchanged,  the  maps  of 
our  Satellite  are  represented  accord- 
ingly. The  four  quadrants  (Plates  23, 
24,  25,  26)  are  designated  in  the  manner 
shown  in  the  annexed  figure.  For  ob- 
servations of  theMoon,  the  "terminator"' 
or  boundary  between  light  and  shade, 
is  the  place  where  the  objects  are  best 
seen,  and  Plates  23 — 38  of  the  present 
Atlas  have  been  arranged  to  facilitate 
observation  of  the  Lunar  formations  on 
the  terminator  at  various  ages,  from  new 
to  full.  The  terminators  for  each  day 
of  a  lunation  are  marked  on  the  quad- 
rants; the  morning  terminator  being 
that  when  the  Sun  is  rising  on  the  ob- 
ject in  question.  The  quadrants  also 
enable  the  latitudes  and  longitudes  of 
Lunar  objects  to  be  found. 

As  the  Moon  is  so  much  more  con- 
veniently observed  from  new  to  full, 
than  from  full  to  new,  it  is  the  former 
series  of  changes  that  have  been  more 
particularly  provided  for.  The  tele- 
scopic view  of  the  Crescent  Moon,  3  days 
old,  is  shown  in  Plate  27.  On  the  oppo- 
site page  an  index  outline  is  given  on 

which  each  of  the  formations  receives  a  special  number  or  letter.  The  name  of  the  formation 
may  be  found  by  looking  out  the  number  or  letter  in  the  Catalogue  of  Lunar  formations  ;  but 
for  greater  convenience  in  reference,  the  names  of  the  chief  objects  visible  in  each  phase  are  set 
out  on  the  Index  outline  as  well.  As  the  Moon  grows  day  by  day,  the  terminator  changes,  and 
an  ever  varying  series  of  objects  is  presented.  A  special  Plate  is  therefore  given  for  each  day 
of  the  Moon's  age,  from  the  3rd  up  to  the  14th,  when  the  Moon  is  full.  Before  the  third  day 
the  Moon  is  so  close  to  the  Sun  that  observations  cannot  be  made  with  advantage. 

Suppose,  for  instance,  that  the  Moon  is  9  days  old.  The  observer  then  refers  to  Plate 
33.  On  the  terminator,  a  little  below  the  middle,  he  notes  a  fine  crater,  and  desires  to  learn 
its  name.  The  Index  outline  assigns  the  Number  380,  and  the  list  on  the  margin  shows  that 
this  feature  is  named  "  Copernicus."  The  observer  will  be  able  to  trace  the  same  object  with 
lessening  detail  up  to  the  time  of  Full  Moon.  See  Plates  34  to  38.  From  the  comparison 
of  any  one  of  these  Plates  with  the  figure  on  this  page,  it  appears  that  Copernicus  must  lie 


THE  MOON.  29 

in  the  "  Second  Quadrant"  or  on  Plate  24,  where  the  great  crater  will  be  found  again  as  No.  380, 
a  conspicuous  object  at  20°  East  longitude,  and  10°  North  latitude.  Along  the  top  of  Plate 
24  are  shown  the  positions  of  the  terminators  at  corresponding  ages  of  the  Moon.  It  will  be 
noted  that  the  morning  terminator  on  the  9th  day  passes  through  Copernicus.  So  also  does 
the  evening  terminator  on  the  24th,  so  that  if  the  observer  desires  to  study  Copernicus  when 
illuminated  by  the  sunlight  from  the  opposite  side,  he  may  repeat  his  observation  15  days  later. 
As  another  illustration,  let  us  suppose  the  Moon  to  be  4  days  old,  and  that  after  com- 
paring the  Moon  with  Plate  28  we  desire  to  know  the  name  of  that  large  round  dark  patch, 
a  little  below  the  centre,  which  lies  midway  between  the  limb  and  the  terminator.  The  Index 
outline  shews  it  marked  A,  and  from  the  reference  to  the  margin  or  to  the  Catalogue  the 
object  is  identified  as  the  Mare  Crisium.  It  is  represented  in  Plate  23  as  A.  near  the  top  at 
the  left. 

To  show  the  mode  of  representing  the  ranges  of  Lunar  mountains,  we  may  suppose  the  stu- 
dent to  be  looking  at  the  Moon  a  little  after  the  first  quarter,  say  on  the  eighth  day,  as  on  Plate 
32.  He  notices  a  remarkable  formation  a  little  below  the  centre.  The  Index  outline  labels 
this  object  c,  and  the  margin  shows  that  we  are  looking  at  the  lunar  Apennines.  Plate  24 
exhibits  the  Apennines  pointing  towards  Copernicus. 

Suppose  that  a  view  of  some  particular  formation  of  known  name  be  specially  desired,  the 
process  is  as  follows.  Look  it  out  in  the  Index  at  the  end  of  this  volume,  the  first  reference 
is  to  the  quadrant,  and  the  next  is  to  the  plate  where  the  object  is  represented  on  the 
terminator. 

Thus,  for  instance,  to  find  the  position  of  Plato.  The  Index  shows  first  of  all  that  it  lies 
on  Plate  24,  that  is,  in  the  Second  Quadrant.  The  next  reference  is  to  Plate  32,  which  shows 
the  object  lying  near  the  terminator  when  the  Moon  is  8  days  old.  There  are  further  references 
to  33,  34,  and  35,  where  the  object  is  also  visible.  The  evening  terminator  on  Plate  24  shows 
that  when  this  object  is  suitably  placed  for  observations  with  the  opposite  illumination,  the 
Moon  is  about  23  days  old.  The  subsequent  references  in  the  Index  are  to  those  pages  of  the 
Introduction  in  which  the  object  is  mentioned. 

The  beginner  should,  however,  be  apprised  that  even  with  the  assistance  which  it  is  hoped 
that  these  maps  will  afford  him,  considerable  pains  are  often  required  to  identify  the  lunar 
objects.  In  the  first  place,  various  causes  produce  what  are  known  as  librations  of  the  Moon, 
whose  effect  is  that  the  Moon  does  not  always  turn  precisely  the  same  face  toward  us. 
The  maps  are  accommodated  to  a  state  of  mean  libration,  and  the  student  must  not  be 
surprised  if  he  finds  an  object  sometimes  higher  and  sometimes  lower  than  its  position  in  the 
map  would  have  led  him  to  expect.  These  changes  often  produce  considerable  variations  in  the 
appearance  of  the  lunar  formations.  It  must  also  be  remembered  that  the  age  of  the  Moon 
cannot  be  always  exactly  that  of  the  map  which  comes  nearest  to  it.  This  will  often  involve 
considerable  alterations  in  the  appearance  of  the  lunar  formations  from  those  which  they  present 
at  the  exact  phase  which  the  map  depicts.  The  elucidation  of  the  several  points  which  thus 
arise  will  afford  much  interesting  occupation,  and  will,  it  is  hoped,  lead  the  student  to  a  close 
acquaintance  with  the  beautiful  scenery  of  our  Satellite. 


30 


POPULAR  GUIDE  TO  THE  HEAVENS. 


CATALOGUE   OF   LUNAR  OBJECTS. 

Figures  refer  to  the  Number  of  the  Crater  or  similar  formation,  capital  letters  refer  to  the  so-called  "  Seas,"  and 
small  letters  refer  to  the  Mountain  Ranges  and  isolated  Mountains. 


1  Langrenus. 

2  Kastner. 

3  Vendelinus. 

4  Maclaurin. 

5  Hecataeus. 

6  Ansgarius. 

7  Petavius. 

8  Wrpttesley. 

9  Palitzsch. 

10  Ease. 

11  Lerendre. 

12  Wilhelra  Humboldt. 

13  Phillips. 

14  Furnerius. 

15  Stevinus. 

16  Snellius. 

17  Adams. 

18  Marinus. 

19  Fraunhofer. 

20  Oken. 

21  Vega. 

22  Pontecoulant. 

23  Biela. 

24  Hagecius. 

25  Boussingault. 

26  Boguslawsky. 

27  Schomberger. 

28  Webb. 

29  Messier. 

30  Lubbock. 

31  Godenius. 

32  Guttemberg. 

33  Magelhaens. 

34  Colombo. 

35  Cook. 

36  Santbech. 

37  McClure. 

38  Crozier. 

39  Bellot. 

40  Borda. 

41  Reichenbach. 

42  Rheita. 

43  Neander. 

44  Metius. 

45  Fabricius. 

46  Janssen. 


47  Steinheil. 

48  Vlacq. 

49  Rosen  berger. 

50  Nearchus. 

51  Hommel. 

52  Pitiscus. 

53  Mutus. 

54  Manzinus. 

55  Censorinus. 

56  Torricelli. 

57  Capella. 

58  Isidorus. 

59  Madler. 

60  Bolmenberger. 

61  Rosse. 

62  Fracastorius. 

63  Piccolomini. 

64  Stiborius. 

65  Riccius. 

66  Rabbi  Levi. 

67  Zagut. 

68  Lindenau. 

69  Nicolai. 

70  Biisching. 

71  Buch. 

72  Hypatia. 

73  Delambre. 

74  Tlieon  Senr. 

75  TlieonJunr. 

76  Taylor. 

77  Alfraganus. 

78  Kant. 

79  Theophilus. 

80  Cyrillus. 

81  Catharina. 

82  Tacitus. 

83  Beaumont. 

84  Descartes. 

85  Abulfeda. 

86  Almanon. 

87  Geber. 

88  Abenezra. 

89  Azophi. 

90  Sacrobosco. 

91  Fermat. 

92  Polybius. 


93  Pons. 

94  Pontanus. 

95  Gemma  Frisius. 

96  Poisson. 

97  Aliacensis. 

98  Werner. 

99  Apianus. 

100  Playfair. 

101  Blanchinus. 

102  La  Caille. 

103  Delaunay. 

104  Faye. 

105  Donati. 

106  Airy. 

107  Argelander. 

108  Parrot. 

109  Albategnius. 

110  Hipparchus. 

111  Ilalley. 

112  Hind. 

113  Horrocks. 

114  Rhseticus. 

115  Reaumur. 

116  Walter. 

117  Nonius. 

118  Fernelius. 

119  Stotler. 

120  Faraday. 

121  Maurolycus. 

122  Barocius. 

123  Clairaut. 

124  Licetus. 

125  Cuvier. 

126  Bacon. 

127  Jacobi. 

128  Lilius. 

129  Zach. 

130  Kinau. 

131  Pentland. 

132  Curtius. 

133  Simpelius. 

134  Miller. 

135  Schubert. 

136  Apollonius. 

137  Firmicus. 

138  Azout. 


THE   MOON. 


31 


CATALOGUE  OF  LUNAR  OBJECTS— continued. 


139  Ne  ;er. 

140  C- aid  ncet. 

141  Belmim. 

142  La  Peyrouse. 

143  Hanno. 

144  Le  Gentil 

145  Tannerus. 

146  lluggms. 

147  Timoleou. 

148  Zeno. 

149  Schwabe. 

150  Ilunsen. 

151  Alhazen. 

152  Picard. 

153  Pierce. 

154  Tarun tins. 

155  Secchi. 

156  Proclus. 

157  Maskelyne. 

158  Jansen. 

159  Vitruviiis. 

160  Maraldi. 

161  Cauchy. 

162  Eininart. 

163  Oriani. 

164  Plutarch. 
Ki5  Seneca. 

166  Macrobius. 

167  Cleomedes. 

168  Tralles. 

169  Burckhardt. 

170  Hahn. 

171  Berosus. 

172  Gauss. 

173  Geuiinus. 

174  Bernouilli. 

175  Messala. 

176  Berzelius. 

177  Hooke. 

178  Schumacher. 

179  Struve. 

180  Mercurius. 

181  Franklin. 

182  Cepheus. 

183  Oersted. 

184  Shuckburgh. 

185  Chevallier. 

186  Atlas. 

187  Hercules. 

188  Endynrion. 


189  De  la  Rue. 

239  Conon. 

190  Strabo. 

240  Mauilius. 

191  Thales. 

241  Ukert. 

192  Gartner. 

242  Triesnecker. 

193  Deiuocritus- 

243  Hyginus. 

194  Arnold. 

244  Agrippa. 

195  Moigno. 

245  Godin. 

196  Peters. 

246  Ritter. 

197  Meton. 

247  Sabine. 

198  Euctemou. 

248  Dionysius. 

199  Challis. 

249  Manners. 

200  Main. 

250  Arago. 

201  Gioja. 

251  Ariadseus. 

202  Scoresby. 

252  Silberschlag. 

203  Barrow. 

253  De  Morgan. 

204  W.  C.  Bond. 

254  Cayley. 

205  Christian  Mayer. 

255  W  he  well. 

206  Archytas. 

256  Calippus. 

207  Aristoteles. 

257  Thesetetus. 

208  Eudoxus. 

258  Cassini. 

209  Alexander. 

259  Aristillus. 

210  Egede. 

260  Autolycus 

211  Great  Alpine  Valley. 

261  Hosting. 

212  Grove. 

262  Lalande. 

213  Mason. 

263  Herschel. 

214  Plana. 

264  Ptolemanis. 

215   Burg. 

265  Alphonsus. 

216   Baily. 

266  Arzachel. 

217   Daniell. 

267  Alpetragius. 

218   Posidomus. 

268  Lassell. 

2l9   Chacornac. 

269  Davy. 

220  Le  Monnier. 

270  Guerike. 

221    Roeiner. 

271  Parry. 

222  Bond. 

272  Bonpland. 

223  Maury. 

273  Fra  Mauro. 

224  Littrow. 

274  Thebit. 

225  Newcoinb.                 \ 

275  Straight  Wall. 

226  Dawes. 

276  Birt. 

227  Plinius. 

277  Purbach. 

228  Ross. 

278  Regiomontanus. 

229  Maclear. 

279  Hell. 

230  Sosigenes. 

280  Pitatus. 

231  Julius  Caesar. 

281  Hesiodus. 

232  Boscovich. 

282  Gauricus. 

233  Taquet. 

283  Wurzelbauer. 

234  Menelaus. 

284  Sasserides. 

235  Sulpicius  Gallus. 

285  Ball. 

236  Bessel. 

286  Lexell. 

237  Linue. 

287  Nasireddin. 

238  Aratus. 

288  Orontius. 

32 


POPULAR  GUIDE  TO  THE  HEAVENS. 


CATALOGUE    OF    LUNAR    OBJECTS— continued. 


289  Pictet. 

290  Saussure. 

291  Tycho. 

292  Heinsius. 

293  Wilhelin  I. 

294  Longomontanus. 

295  Street. 

296  Maginus. 

297  Deluc. 

298  Clavius. 

299  Cysatus. 

300  Moretus. 

301  Short. 

302  Newton. 

303  Gruemberger. 

304  Cabeus. 

305  Casatus. 

306  Klaproth. 

307  Wilson. 

308  Kircher. 

309  Bettinus. 

310  Zuchius. 

311  Segner. 

312  Blancanus. 

313  Schemer. 

314  Weigel. 

315  Rost. 

316  Bailly. 

317  Schiller. 

318  Bayer. 

319  Pingre. 

320  Hausen. 

321  Phocylides. 

322  Wargentin. 

323  Schickard. 

324  Drebbel. 

325  Inghirami. 

326  Hainzel. 

327  Lehmann. 

328  Lacroix. 

329  Piazzi. 

330  Lagrange. 

331  Fourier. 

332  Vieta. 

333  Doppelmayer. 

334  Lee. 

335  Vitello. 

336  Clausius. 

337  Capuanus. 

338  Cichus. 


339  Mercator. 

340  Campanus. 

341  Kies. 

342  Bullialdus. 

343  Lubiniezky. 

344  Nicollet. 

345  Hippalus. 

346  Agatharchides. 

347  Gassendi. 

348  Herigonius. 

349  Letronne. 

350  Mersenius. 

351  Cavendish. 

352  Byrgius. 

353  Eichstadt. 

354  De  Viccr. 

355  Ramsden. 
35b  Billy. 

357  Hansteen. 

358  Sirsalis. 

359  Fontana. 

360  Zupus. 

361  Cruger. 

362  Rocca. 

363  Grimaldi. 

364  Damoiseau. 

365  Riccioli. 

366  Lohrmann. 

367  Hermann. 

368  Flamsteed. 

369  Wichmann. 

370  Euclides. 

371  Landsberg. 

372  Gambart. 

373  Sommering. 

374  Schroter. 

375  Pallas. 

376  Bode. 

377  Reinhold. 

378  Hortensius. 

379  Milichius. 

380  Copernicus. 

381  Stadius. 

382  Eratosthenes. 

383  Gay  Lussac. 

384  Tobias  Mayer. 

385  Kunowsky. 

386  Encke. 

387  Kepler. 

388  Bessarion. 


389  Reiner. 

390  Marius. 

391  Hevel. 

392  Cavalerius. 

393  Gibers. 

394  Cardanus. 

395  Kraftt. 

396  Vasco  de  Gania. 

397  Seleucus. 

398  Marco  Polo. 

399  Archimedes. 

400  Beer. 

401  Timocharis. 

402  Lambert. 

403  Pytheas. 

404  Euler. 

405  Diophantus. 

406  Delisle. 

407  Caroline  Herschel. 

408  Carlini. 

409  Leverrier. 

410  Helicon. 

411  Kirch. 

412  Piazzi  Smyth. 

413  Plato. 

414  Tinmis. 

415  Birmingham. 

416  Epigenes. 

417  Goldschmidt. 

418  Anaxagoras. 

419  Fontenelle. 

420  Philolaus. 

421  Anaximenes. 

422  J.  J.  Cassini. 

423  Condamine. 

424  Maupertuis. 

425  Bianchini. 

426  Sharp. 

427  Mairan. 

428  Foucault. 

429  Harpalus. 

430  J.  F.  W.  HerscheL 

431  Anaximander. 

432  Pythagoras. 

433  South. 

434  Babbage. 

435  (Enopides. 

436  Robinson. 

437  Cleostratus. 

438  Xenophanes. 


THE   MOON. 


33 


CATALOGUE  OF  LUNAR  OBJECTS— continued. 


439  Repsold. 

440  Harding. 

441  Gerard. 

442  Lavoisier. 

443  UlughBeigh. 

444  Lichtenberg. 


445  Briggs. 

446  Otto  Struve. 

447  Aristarchus. 

448  Herodotus. 

449  Wollaston. 

450  Schiaparelli. 


451  Gruithuisen. 

452  Brayley. 

453  Galileo. 

454  Horrebow. 


MOUNTAIN    RANGES  AND   ISOLATED   MOUNTAINS. 


a    Alps. 
b     Caucasus. 
c     Apennines. 
d    Carpathians. 

e     Sinus  Iridum  Highlands. 

/    Hsemus. 

g    Pyrenees. 

h    Altai  Mountains. 

t     Riphsean  Mountains. 

j     La  Hire. 

'k    Mt.  Taurus. 

I     TeneriS'e  Range. 

Mountains  near  the  Limb  : — 

D'Alembert  Mts. — on  the  east  limb,  extending  from  S.  lat.  19°  to  N.  lat.  12°. 
The  Cordilleras — near  the  east  limb,  extending  from  S.  lat.  23°  to  S.  lat.  8°. 
The  Rook  Mountains— on  the  east  limb,  extending  from  S.  lat.  39°  to  S.  lat.  16°. 
The  Doerfel  Mountains — on  the  south-east  limb,  extending  from  S.  lat.  80°  to  S.  lat.  57° 
The  Leibnitz  Mountains  extend  from  S.  lat.  70°  on  the  west  limb  to  S.  lat.  80°  on  the 

east  limb. 
Humboldt  Mountains— on  the  west  limb,  extending  from  N.  lat.  72°  to  N.  lat.  53°. 


m   Straight  Range. 
n    Percy  Mountains. 
o     Harbinger  Mountains. 

Hercynian  Mountains. 

Pico. 

Piton. 

Mt.  Argaeus. 

Mt.  Hadley. 

Laplace  Promontory. 

Mt.  Huygens. 

Mt.  Bradley. 


MARIA  or  SEAS 


A  Mare  Crisium. 

B        ,     Fcecunditatis. 

C        ,     Australe. 

D        ,      Humboldtianum. 

E        ,      Tranquillitatis. 

F        ,     Nectaris. 

G  Lacus  Somniorum. 

H       „     Mortis. 

J  Mare  Serenitatis. 

K        „     Frigoris. 

L        „     Imbrium. 

M  Vaporum. 


N  Sinus  JEstuum. 

P        „     MediL 

Q  Mare  Nubium. 

R  Sinus  Iridum. 

S  Oceanus  Procellarurn. 

T  Mare  Humorum. 

V  Palus  Somnii. 

W  Sinus  Roris. 

X  Palus  Nebularum. 

Y  Mare  Smythii. 

Z  Palus  Putredinis. 


BALLS  POPULAR  GUIDE  TO  THE  HEAVENS 


Key  to  Plate  20 


AJfraganus 


Taylor 


Sabine, 


O  Tkeon.  Jim. 
0  ZTuwn,  S«v 


Bitter 
O        O  Dionysiu* 


Arago 


(     JGodin, 
Agrtppa, 


Sosigeries     ^J     ( 


p 

Janten. 


0 


Boscoviah, 


O 


SERENITATIS 


Cononi 

M 

.  <v 
-  '*<? 


PosZoLora 


Archimedes 


UtotxCOS  J-— '—-v 


AristMiLS 


tit's   POPULAR  GUIDE  TO  THE    HEAVENS. 


Plate  20. 


'••  •  > 


•        ,>  . 

•••Pf  ^K*****  • 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS 

I 


Mo  i 
"<^_ 

X 
Cysati. 


o 


'o 


SCLUSSUJV 


(9. 


\  WtUiebn 


V 
oO, 


O 

O 


O  o 


Wurz  eii>  cajucr 


Key  to  Plate  21 


-Merfajtor 
J 


Plate  21. 


.V-H  «       W*& 


THE  MOON.   REGION  OF  CLAVIUS  AND  TYCHO.     G.  W.  RITCHEY,  40-in.  Refractor.  YERKES  OBSERVATORY. 


Key  Map. 


THE  MOON— 3rd  Day. 


To  face  Plate  27. 


25.  Bovssingault. 
22.  Ponttcoulant. 
19.  Fraunhofer. 


14.     Furnerius. 
12.     W.  Humboldt. 
7.    Petavius. 


3rd  Day 


A  Mare  Crisium. 

B        ,,      Facunditatis. 

C        ,,     Australe. 

D        ,,      Hvmboldtianum . 


1 1.  Legendre. 

5.  Hecataus. 

3.  Vendelinus. 

I.  Langrenus, 

z.  Kastner. 

136.  Apollonius. 

137.  Firmicus. 

139.  Neper. 

140.  Condor cet. 

152.  Picard. 
151.  AiAaten. 

153.  Peirce. 
167.  Cleomedes. 
169.  Burckhardt. 
173.  Geminus. 
172.  Gauss. 

175.  Messala. 

180.  Mercurius. 

188.  Endymion. 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS.      3*?    DAY. 


Plate    27. 


Key  Map. 


THE  MOON— 4th  Day. 


To  face  Plate  28. 


26.  Boguslawsky. 
25.  Bovssingault. 
22.  Pontlcoulant. 


49.     Rosenberger. 
46.    Janssen. 
45.     Fabricivs. 


A  Mare  Crisium. 

B  ,,      Faecunditatis. 

C  ,i       Australe. 

D  i>      Humboldtianum. 


44.  Metius. 

14.  Furnerivs. 

15.  Stevinui. 
43.  Neander. 
41.  Reichenbach. 
1  6.  Snellius. 

7.  Petavius. 

36.  Santbech. 

34.  Colombo. 

3.  Venddinuf. 

31.  Godenius. 

32.  Guttcmberg. 
i.  Langrenvs. 

30.  Lubbock. 

29.  Messier. 

155.  Secchi, 
136.  Apollonius. 
154.  Taruntius. 

152.  Picard. 

153.  /V*r«. 

156.  Proclus. 

1  66.  Macrobius. 

167.  Cleomedes. 

169.  Burckhardt. 

173.  Gauss. 

176.  Beruelius. 

175.  Messala. 

181.  Franklin. 

185.  Chevaliier. 

186.  yi//aj. 
180.  Mercurius. 
188.  Endymion. 
189. 


V    /'a/aj  Somnii. 


g.     Pyrenees  Mis. 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS.      4-T?    DAY. 


Plate      28. 


George  Ph-dip  A  Son.,L*? 


Key  Map. 


THE  MOON— 5th  Day. 


To  face  Plate  29. 


27.    Schombcrger. 
54    Manzinus, 
53.     Mittus. 


49.     Rosenberger. 
48.      Vlacq. 
52.     Pitiscus. 


A  Mare  Crisium. 

B  ,,      Ftecunditatis. 

E  ,,       Tranquillitatis. 

F  ,,       Nectans. 


D     Mzr*  Hvmboldtianum. 
G     Locus  Somniorvm. 
H  Mortis. 


46.    Janssen. 
45.    Fabricius. 
44.     Metius. 
14.     Fumtrius. 
64.     StiboHus. 
63.     Piccolomini, 
62.    Fracastorius. 
36.     Santbech. 

7.    Petavius. 

3.     Vendelinui. 

31.  Godenius. 

32.  Guttemberg. 
I.     Langrenvs. 

58.     Isidorus. 

57.     Capella. 

29.     Messier. 

159.      Vitruvius. 

220.  Z>  Monnier. 

221.  Roemer. 
219.     Chacornac. 
224.     Littrow. 
218.     Posidonius. 
214.     Plana. 

187.  Hercules. 
186.     ^4//aj. 

188.  Endymion. 

189.  Z>*  Za  ^a^. 
198.     Euctemon, 


g.    TA^  Pyrenees. 
k.     Taurus  Mts. 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS. 


TV    DAY. 


Plate    29. 


UNIVERSITY 

OF 


Key  Map. 


THE  MOON— 6th  Day. 


To  face  Plate  30. 


126.     Bacon. 
121.     Maurolycus. 


71.     Buck. 

64.  Stiborius. 

65,  Riceius. 


A  jl/ar«  Crisium. 

B  ,,      Fcecunditatis. 

D  ,,      Humloldtianvm. 

E  ,,      Tranquillitatis. 


F  A/are  Nectaris. 

G  Locus  Somniorum. 
H         „      Mortis. 

J  Mare  Serenitatis. 


66. 

67. 

63- 

93- 

62. 

90. 

91. 

81. 

82. 

80. 

59- 

79- 

72. 
247. 
246. 
250. 
229. 
228. 
227. 
226. 
236- 
220. 
219. 
218. 
208. 
207. 
197. 
202. 


Rabbi  Levi. 

Zagvt. 

Piccolomini . 

Pans. 

Fracastorius. 

Sacrobosco. 

Per  mat. 

Catharina. 

Tacitus, 

Cyrillus. 

Madler. 

Theophilus. 

Hypatia, 

Sabine. 

Ritter. 

Arago. 

Maclear. 

Ross. 

Plinius. 

Dawes. 

Bessel. 

Le  Monnier. 

Chacornac. 

Posidonius. 

Eudoxus. 

Aristoteles. 

Meton. 

Scoresby. 


g.     The  Pyrenees. 
h.     Altai  Mts. 
k.     Taurus  Mts. 
8.   Mt.  Argaeus. 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS.       6T?    DAY. 


Plate    30. 


GeargeP/uhp  &  Son.L' 


Key  Map. 


THE  MOON— 7th  Day. 


To  face  Plate  3l. 


132.     Curtius. 
129.     Zach. 
128.     Lilivs, 


124. .  Licetus. 
123.     Clairaut. 
121.     Maurolycus. 


A  A/a^-tf  Crisium. 

B  ,,      Fcecunditatis. 

E  „      Tranqttillitatis. 

G  £ac«j  Somniorum. 


F  A/artf  Nfctaris. 
K  „  Frigoris, 
M  „  Vaporum* 
J  ,,  Strenitatis. 


119.     StSfler. 
118.     Fernelius. 

97.  Atiacensis. 

98.  Werner. 

99.  Apianus. 
100.    Playfair. 
xoi.     Blancfiinus. 

105.  Donati. 

106.  ^»/y. 

86.     Almanon* 
85.     Abulfeda. 

109. 

ita. 

in.     Halley. 

no.    Hipparckus. 

113.  HorrocJu. 

114.  Rhaelicus. 
245.     Godin. 
244.     Agrippa. 
242.     Triesnecker. 

232.  Boscovich. 
231.    Julius -Caesar. 
240.     Manilius. 

233.  Taquet. 

234.  Menelaus. 

236.  5wjtf/. 

237.  Linnet. 
257.     Thecetetu*. 
256.     Calippus. 
209.    Alexander. 
208.     Eudoxus. 
207.     Aristotetes. 

H  Locus  Mortis. 

X  /Wwj  Nebularum, 

M  A/ar«  Vaporum. 

b«  7/i*  Caucasus, 

f.  The  Heemus  Mts. 

k.  rAr  Tizaraj  J//j. 


BALLS  POPULAR  GUIDE  TO  THE  HEAVENS.        7T?   DAY. 


Plate   31. 


Tiden  GwgraphicailrisUtate. . 


Key  Hap. 


THE  MOON— 8th  Day. 


To  face  Plate  32. 


300.     Moretus. 
297.     Deluc. 
296.    Maginus. 


290.     Saussure. 
116.     Walter. 
285.     Ball. 


P  Sinus  Medii. 

M  ,,     sEstuum. 

J  A/ar«  Serenitatis. 

L  ,,     Imbrium, 


X  /Witf  Nebularum. 
2.  ,,  Putredinis. 
K  A/«r«  Frigoris. 


278.     Regiomontanus. 
277.     Purbach. 
274.     7"A<rfoV. 

266.  Arzachel. 

267.  Alpetragius. 
265.  Alpkonsus. 
264.  PtolemcBt'.s. 
263.  Herschel. 
261.  Masting. 

373.  Sbmmering. 

374.  Schroter. 
376.  2?<Mfe. 
260.  Autolycus. 
399.  Archimedes. 

257.  Tkecetetus. 
259.  Aristillus. 

258.  Cassini. 

211.  Great  Alpine  Valley. 

413.  P/afe. 

414.  Timaus, 

416.  E pi  genes. 

417.  Goldschmidt. 


b.  T'A*  Caucasus. 

c.  7Vi<  Apennines. 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS.       8TV    DAY. 


Plate    32. 


Georyeffuiy  6tSon,L'? 


TTnLondon,  Geographical.  Institute 


«" 

OF  THE 


OF 


• 


Key  Map. 


THE  MOON— 9th  Day. 


To  face  Plate  33. 


300.  Moreius. 
299.  Cysatus. 
298.  Clavius. 


296.    Maginus. 

294-     Longomontanus. 

291.     Tycho. 


Dav 


K    Afare  Frigoris. 

L        „      Imtrium. 
N 


P    Sinus  A/edit. 
Q    J/ore  NvHum, 


293. 

292.     Heinsiui. 
282.     Gauricus. 

2^3.  Wurtelbauer. 

280.  Sasscridcs. 

281.  Hcsiodus. 
275.  Straight  Wall. 
344.  Nicollft, 

274.  Thebit. 

266.  Artacht:. 

267.  Alfetragiiu. 
265.  Alpkoitsus. 
264,  Ptolcmtfus. 

270.  Guerike. 

271.  Parry. 
2/3.  Bonpland.^ 
262.  Lalande. 
273.  /•>»  Mauro. 
372.  Gambart. 
377.  Reinhold. 

381.  Stadius. 
380.  Copernicus. 

382.  Eratosthenes. 

383.  Cox  LMSSOC. 
403.  Pytheas. 

401.  Titnocharis. 

402.  Lambert. 
399.  Archimedes. 
413.  /'^fe. 

419.  Fcmienelle. 

417.  Goidschmidt. 

418  Anaxagoras. 


a. 

b.  7*^  Caucasus  Mta. 

c.  The  Apennines. 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS. 


T?    DAY. 


Plate     33. 


Tfi&Lnndon  G&ographi-cai  Institute 


Key  Map. 


THE  MOON— 10th  Day. 


To  face  Plate  34. 


302.     Newton. 

312.  Bla.ncan.us. 

313.  Sckeiner. 


K  Mare  Frigoris. 
L        , ,      Imbrium, 

P  Sinus  Medii. 

Q  *Mare  Nubium. 

R  Sinus  Jridum, 


298.     Clavius.                                         2gI 

Tycho. 

315.     Host.                                               293. 

Wilhelm  I. 

294.     Longomonianus.                             292. 

Heinsius. 

326. 

Hainsel. 

338. 
rr'7SS^                                  337- 

Cichus. 

Capuanus. 

Oz*\                           355' 

Ramsden. 

231     <^>2.9A\                                       339- 

0    0  (T)     ,  o                   r,2C 
340. 

Merc  a  tor. 
Campanus. 

Q  Q  333    (\3^                              341' 

Kies. 

•fcu. 

^  —  —    ^  >  *-^>         J35  Ci            ^                                OTW 

Hippalus. 

342. 

O^^iV    \                  ^4^' 
P)                                                  M 

/TO        ^                        370. 

Bullialdus. 
Lubiniezky. 
Euclides. 

0                ty     N           371- 
U                 Occx           0^7,          3,/7_ 

380. 

Landsberg. 
Rcinhold. 
Copernicus.  • 

(^  p                                                           384 

Tobias  Mayer. 

V^                          ^"?                   383- 

X  // 
/       o       O580                403- 

<  Q       /  o^       *84  1           402- 

Gay  Lussac. 
Pytheas. 
Lambert. 

0  /^"                                **   d              1                    404. 

Euler. 

40i. 

Timocharis. 

,                 O^<92.     *^     /                      --^ 
c->                                                                      T(yw» 

r\       *"                       f** 

Archimedes. 

407. 

Caroline  Herschel, 

409. 

Leverrier. 

4a90/J420 
V.                              ,                    w/l       ~J 

Helicon. 

^-£-'^^~~^~  S~^)                                                                             ^^' 

Plato. 

433- 

Condamirte. 

419. 

Fontenelle. 

420. 

Philolaus. 

a.     The  Alps. 

b.     The  Caucasus.                              r.    / 

c.     The  Apennines.                              U.    / 

>rom.  Laplace. 

d.     The  Carpathians. 

q- 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS.        IOT"    DAY. 


Plate    34. 


Tke,Londoru  Geographical,  fasUln 


Key  Map. 


THE  MOON— llth  Da5 


To  face  Plate  35. 


305.  Casatus. 

306.  Klaproth. 

307.  Wilson. 


309-     Bettinus. 
311.     Segner. 
313.     Scheiner. 


L  Mare  Imbrium. 

K        ,,      Frigoris. 

Q        „      Nubium. 

R  Sinus  Iridum. 

S  Oceanus  Procellarum. 

T  .d/are  Humorum. 


b.  TA*  Caucasus. 

c.  7^<?  Apennines. 

d.  7"A«  Carpathians. 

e.  7"Ag  Sinus  Iridum 

Highlands. 


298.     Clavius. 
315.     AW. 

317.  Schiller. 

318.  Bayer. 
326.     Hainzel. 
337.     Capuanus. 
355.     Ramsden. 
335-     Vitello. 
334.     £<•*. 

333.     Doppelmayer. 

345.  Hippalus. 

346.  Agatharchides. 

347.  Gassendi. 

348.  Herigonius. 

349.  Letronne. 

370.  Euclides. 
369.      Wichmann. 
368.     Flamsteed. 

371.  Landsberg. 

385.  Kunowsky, 

377.  Reinhold. 

386.  Encke. 

378.  Hortensius. 

387.  Kepler. 
380.     Copernicus. 

379.  Milichius. 
384.     Tobias  Mayer. 
383.     Gaj  Lussac. 

404.  Euler. 
4°2-     Lambert. 
401.      Timocharis. 
399.     Archimedes. 

405.  Diophantus. 

406.  Delisie. 
451.     Gruithuisen. 
427.     Mairan. 
426.    Sharp. 

425-     Bianchini. 

423.     Condamine. 

413-     />/a/^. 

429.     Harpalus. 

43°.     /.  -F.  IF.  Herschel. 

420.     Phitolaus. 

421-     Anaximenes. 

i.      Riphaan  Mis. 

q.     /'jVo. 

r.     Piton  Mountain. 

U.    Prom.  Laplace. 


BALL'S  PbPULAft  GUIDE  TO  THE  HEAVENS.         MT?  DAY. 


Plate  35. 


ffeoryefhdtf  Sr  Son,!,'? 


Key  Map. 


THE  MOON— 12th  Day. 


To  face  Plate  36. 


317.  Schiller, 

318.  Bayer. 
321.    Phocylidei. 


323.  Schickard. 
327.  Lthmann. 
332.  Vieta. 


Day 


351.     Cavendish. 
350.     Mersenius. 
347.     Gassendi. 
359.     Fontana. 

356.  5*Y/y. 

357.  Hansteen. 
349.     Letronne. 
368.     Flamsteed. 
386.     .£«£&;. 
389     Reiner. 
387      .«r/A?r. 
390.     Marius. 

447.  Aristarchus . 

448.  Herodotus. 
404.     Euler. 

449.  Wollaston. 
427.     Mairan. 
426.     Sharp. 
425.     Bia.nch.ini. 
423.     Condamine, 

419.  Fonteneile. 
431.     Anaximander. 

420.  Philolaus. 

421.  Anaximenes. 


K  Mare  Frigoris. 
L  „  Imbrium. 
Q  „  Nubium. 


S     Oceanus  Procellariim. 
T     Mare  Humor-urn. 


e.     The  Sinus  Iridum 

Highlands. 
H.    Prom.  Laplace. 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS.         I2T.H    DAY. 


Plate  36. 


ThzLoncLan.  Gs^yraphiccdhsUtate 


Key  Map. 


THE  MOON— 13th  Day 


To  face  Plate  37. 


321.     PkocyKdes.                                          ^     Schickard.                                                332. 

Plfte, 

322.      Wargentin.                                        339.    />,•<,„,-.                                                       35*- 

Cavendish. 

325.     Inghirami.                                         33O     Lagrange.                                                  35°. 

Merseniui. 

raatn                                                                                                                            3S9- 
73  ,,  Da\ 

361- 

Fontana. 
Criiger. 

362. 

Rocca, 

363- 

Grimalai. 

366. 
V  S?^f  aj 

Lohrmann. 

391- 

"   ^     \                       392. 
[  toy.9 

1                                            397- 

Hevel. 
Cavalerius. 
Seleucus. 

V        (}  K  -*  /-7   \                           44-8. 
1            ^            \   0     ^              \ 

Herodotus. 

447- 
445- 

•rj  ^  \  r*  432. 

Aristarchus. 
Briggs. 
Pythagoras. 

L  Mare  Imbrivm. 
Q  .,  Nttbium. 
R  Sinus  Iridum. 


S    Oceanus  Procellarttm. 
T    Mare  Humorum, 
W    5m«j  Roris. 


6.     TA/  5r«wJ  Iridum 

Highlands. 
u.    Prom.  Laplace. 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS.        13-™  DAY 


Plate  37. 


Key  Map. 


THE  MOON— ,14th  Day. 


To  face  Plate  38. 


316.     Bailly. 

321.  Pkocylides. 

322.  Wargentin. 


K  A/rtre  Frigoris. 
L  Afar<?  Imbrium. 
R  5*'««j  Iridum. 


32S-  Inghirami. 
323.  Schickard. 
329.  Piazzi. 


S     Oceanus  Procellarum. 
T    A/ar«  ffumorum. 
W   5»«»j  Roris. 


330.     Lagrange. 
353.     Eiekstadt. 

362,  Rocca. 

363.  Grimaldi. 

365.  Riccioli. 

366.  Lohrmann. 

391.  Hevel. 

392.  Cavalerius. 
393-     0/forj. 
394.     Cardanus. 
395-     Kraft. 
448.     Herodotus. 
447-     Aristarchus. 
397-     Seieucus. 
445-     Briggs. 
446.     0#0  Struve. 
439.    Repsold. 
438-     Xenophancs. 
437-     Cleostratus, 
432.     Pythagoras. 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS. 


Plate   38. 


GtergePhdif  &  Son^L'f 


ThiLomion,  GtoanwhicaL  Institute 


UNIVERSITY 

or 


(35) 


CHAPTER    VI.— THE  SKY  MONTH  BY  MONTH,  AND   THE 
INDEX   TO   THE  PLANETS. 

PLATES  39—50. 
THE    MONTHLY    MAPS. 

The  diurnal  rotation  of  the  earth  gives  rise  to  an  apparent  revolution  of  the  celestial 
sphere  in  a  period  of  one  sidereal  day.  In  consequence  of  this  movement  the  appearance  of 
the  sky  is  continually  changing,  so  that  to  the  beginner  it  is  often  a  matter  of  considerable 
difficulty  to  know  where  to  look  for  any  particular  star  or  constellation. 

As  the  sidereal  day  is  about  4  minutes  shorter  than  the  ordinary  mean  solar  day,  the  effect 
produced  is  a  gradual  shifting  of  the  stars  from  east  to  west, — a  star  which  occupies  a  certain 
position  one  night,  reaches  the  same  position  4  minutes  earlier  the  next  night,  so  that  at  the 
end  of  a  month  this  position  is  attained  2  hours  earlier  than  at  the  beginning. 

In  order  to  render  these  changes  easier  to  follow,  and  to  enable  the  student  to  identify 
the  principal  constellations  without  difficulty,  and  to  know  where  any  particular  star  or 
group  of  stars  is  to  be  found  at  any  time,  Plates  39 — 50  are  used.  They  represent  the  positions 
of  the  principal  stars  down  to  the  4th  magnitude  at  intervals  of  2  sidereal  hours.  The  first 
shows  the  aspect  of  the  heavens  at  midnight  on  January  15th,  the  sidereal  time  then  being  7h. 
37m.  This  map  will  also  represent  the  appearance  of  the  visible  hemisphere  at  the  times 
shown  in  the  corners  at  the  top.  Thus  we  find  that  the  first  of  the  monthly  maps  may  be  used 
in  February  at  10  P.M.,  in  March  at  8  P.M.  From  April  to  September  inclusive,  the  stars  will 
occupy  the  positions  here  indicated  during  the  daylight  hours,  when  they  will  be  invisible  ;  but 
in  October  this  aspect  of  the  sky  may  again  be  seen  at  6  A.M.,  in  November  at  4  A.M.,  and  in 
December  at  2  A.M.  To  find  the  right  map  for  any  month  and  hour  we  can  make  use  of  the 
following  Table. 

TABLE  TO  FIND  THE  ASPECT  OF  THE  HEAVENS  AT  ANY  GIVEN 
MONTH  AND  HOUR  OF  NIGHT. 


P.M. 
4h. 

P.M. 

6h. 

P.M. 

8h. 

P.M. 

10h. 

Mid- 
night. 
12h. 

A.M. 

2h. 

A.M. 

4h. 

A.M. 

oh. 

A.M. 

8h. 

January.... 
February  .  . 
March  

47 

48 
49 
50 

49 
50 
39 

50 
39 

40 

39 

40 
41 

40 
41 
42 

41 
42 
43 

42 
43 
44 

43 

April  
May  

40 
41 

41 
42 

42 
43 

43 

44 

44 
45 

June  

42 

43 

44 

45 

46 

July  

43 

44 

45 

46 

47 

August  .... 
September. 

44 

44 
45 

45 
46 

46 

47 

47 
48 

48 
49 

50 

October  
November- 
December.  . 

45 
46 

45 
46 

47 

46 

47 
48 

47 
48 
49 

48 
49 
50 

49 
50 
39 

50 
39 
40 

39 

40 
41 

41 
42 

36  POPULAR  GUIDE  TO  THE  HEAVENS. 

EXAMPLES  OF  THE  USE  OF  THIS  TABLE  : 

I.  To  find  the  map  suitable  for  10  P.M.  in  March.    Take  the  third  row,  and  under  10  P.M 

is  found  40.    This  means  Plate  40. 

II.  What  map  should  be  used  at  7-30  P.M.  in  November  ?    On  the  eleventh  row  we  find 

47  under  8  P.M.,  and  as  7-30  is  nearer  to  8  than  to  6,  we  accordingly  choose  Plate  47. 

It  will  of  course  be  understood  that  the  maps  have  been  designed  to  represent  the  appear- 
ance of  the  sky  on  the  15th  of  each  month,  at  the  hours  mentioned.  The  changes  are,  however, 
so  slow,  that  for  most  purposes  they  will  be  found  sufficiently  applicable  to  the  whole  month. 
If,  however,  greater  precision  is  desired,  it  can  be  obtained  by  subtracting  half-an-hour  from 
the  time  given  on  the  map  for  each  week  after,  or  adding  half-an-hour  for  each  week  before 
the  middle  of  the  month.  Plate  39  is  thus  quite  accurate  at  11  P.M.  on  the  30th  of  January, 
or  at  8-30  P.M.  on  March  8th  ;  similarly  Plate  41  is  correct  at  5  A.M.  on  the  30th  December,  or 
at  1 1  P.M.  on  the  1st  April. 

These  maps  have  been  constructed  for  the  latitude  53°  23'  N.  They  will  thus  be  suitable 
for  all  parts  of  the  British  Isles.  The  bounding  circle  represents  the  horizon,  and  the  small 
cross  at  the  centre  marks  the  position  of  the  zenith.  The  projection  used  is.  such  that  the 
distance  of  each  star  from  the  centre  of  the  map  is  proportional  to  the  star's  zenith  distance. 
The  angle  which  the  line  joining  a  star  to  the  zenith  makes  with  the  central  meridian  is  the 
azimuth  of  the  star. 

As  the  celestial  sphere  is  viewed  from  the  inside,  the  cardinal  points  are  not  disposed  on 
these  maps  as  in  a  terrestrial  atlas.  In  the  present  case,  when  the  north  is  at  the  top,  the  west 
will  be  on  the  right,  and  the  east  on  the  left. 

If  we  wish  to  compare  any  region  of  the  sky  with  the  map,  we  suppose  a  radius  drawn 
through  the  middle  of  this  region,  and  the  point  where  it  cuts  the  circumference  of  the 
map  gives  us  the  azimuth.  Turning  our  face  towards  this  point  of  the  compass,  we  hold 
the  map  so  that  the  corresponding  point  of  the  circumference  is  lowest,  and,  remembering 
that  the  centre  of  the  circle  represents  the  zenith,  we  have  on  the  map  a  picture  of  the  corres- 
ponding position  of  the  sky.  For  instance,  if  we  wish  to  find  the  constellation  Leo  at  midnight, 
in  the  middle  of  March,  we  find  from  Map  41  that  the  radius  drawn  through  the  middle  of  it 
cuts  the  circumference  at  about  fths  of  the  way  from  south  towards  west.  We  accordingly  turn 
to  that  point  of  the  horizon,  and  we  can  readily  find  this  constellation.  The  brightest  star, 
Regulus,  will  be  then  almost  exactly  half-way  between  the  zenith  and  horizon,  or  at  an  altitude 
of  45°,  while  the  "  Sickle,"  which  forms  the  fore-part  of  this  constellation,  will  be  found  tilted 
over  towards  the  west.  If  we  turn  a  little  further  towards  the  west,  we  shall  find  the  two 
bright  stars,  Castor  and  Pollux,  a  little  lower  in  the  sky,  the  line  joining  them  being  nearly 
horizontal.  Again,  if  in  October,  at  10  P.M.,  we  wish  to  examine  "  The  Plough,"  as  the  group 
formed  by  the  principal  stars  in  Ursa  Major  are  called,  we  go  to  Plate  47  and  find  this  group 
almost  due  north.  We  have  accordingly  to  turn  the  map  upside  down,  so  as  to  bring  the  north 
point  lowest,  and  we  then  see  this  figure  stretching  across  the  sky  in  a  horizontal  direction,  at 
an  altitude  of  about  20°.  If  we  turn  to  the  north-east,  we  again  find  Castor  and  Pollux  just 
above  the  horizon,  but  this  time  the  line  joining  them  is  very  nearly  vertical. 

The  names  of  the  constellations  have  been  printed  on  the  maps,  so  that  when  the  maps 
are  held  in  the  proper  position  for  any  constellation  its  name  may  be  erect. 

As  the  student  becomes  more  familiar  with  the  stars,  he  will  probably  wish  to  identify 
many  fainter  groups  that  do  not  appear  on  these  plates.  In  order  to  enable  this  to  be  done 


THE  SKY  MONTH  BY  MONTH.  37 

with  facility,  the  faint  dotted  lines  have  been  inserted  which  mark  the  boundaries  of  the 
regions  which  each  of  the  plates,  51 — 70,  of  the  general  Atlas,  cover  on  the  sky.  These  lines 
appear  everywhere  in  pairs,  the  spaces  between  the  pairs  being  the  areas  by  which  the 
maps  overlap  each  other.  The  numbers  within  the  regions  thus  marked  out  are  those  of  the 
corresponding  plates  in  this  volume,  where  more  detailed  maps  of  the  same  part  of  the  sky 
will  be  found.  Thus  in  Plate  40  we  find  the  constellation  Leo  almost  wholly  contained  in 
the  space  corresponding  to  Plate  60,  and  some  of  its  principal  stars  in  the  space  common  to 
Plates  54,  55,  and  60.  If  we  turn  to  Plate  60  we  shall  find  the  whole  group  on  a  much  larger 
scale,  while  54  and  55  show  the  more  northern  parts  of  this  constellation. 


THE    INDEX   TO    THE    PLANETS. 

It  is  a  special  object  of  this  work  to  indicate  from  month  to  month  and  from  year  to 
year  the  positions  in  the  sky  of  the  principal  planets.  Let  it  be  once  for  all  understood  that 
those  who  want  exact  positions  must  seek  for  them  elsewhere — in  the  Nautical  Almanac  for 
example.  What  is  here  given  is  an  in  flex  to  the  planets  sufficient  for  the  following 
purposes : 

(1)  To  find  the  place  in  the  heavens  which  each  principal  planet  occupies  in  each  month 
of  the  years  1901—1950. 

(2)  To  find  when  any  principal  planet  rises,  or  souths,  or  sets. 

(3)  To  determine  thence  the  best  season  of  any  year  for  the  observation  of  the  planet. 

(4)  To  find  the  name  of  a  planet  when  the  time  and  place  of  its  appearance  are  known. 

The  following  table  gives  the  dates  of  the  Planetary  Phenomena  described  at  the  head 
of  each  column. 


PLANETARY   PHENOMENA. 


1 

2 

3 

4 

5 

6 

7 

8 

9 

MERCURY. 

Vesus. 

Phase 

Greatest  Elongation. 

liars 

Jupiter 

Saturn 

of 

A.D. 

in 

in 

in 

Saturn 

Evening  Star. 

Morning  Star. 

Evening. 

Morning. 

Opposition. 

Opposition. 

Opposition. 

Plate  8 

1901 

Feb.  16—23 

July  29—  Aug.  5 

December 

February 

June 

July 

7 

1902 

Jan.  30—  Feb.  6 

Oct.  31—  Nov.  7 

— 

April 

— 

August 

July 

7 

1903 

May  5—12 

Oct.  15—22 

July 

November 

March 

September 

July 

8 

1904 

Apr.  18—25 

Sept.  28—  Oct.  5 

— 

— 

October 

August 

8 

1905 

Mar.  30—  Apr.  6 

Sept.  11—  18 

February 

July 

May 

November 

August 

8 

1906 

Mar.  14—21 

Aug.  26—  Sept.  2 

September 



— 

December 

September 

9 

1907 

Feb.  26—  Mar.  5 

Aug.  9—16 

— 

February 

June 

— 

September 

9 

1908 

Feb.  9—16 

Nov.  10—17 

April 

September 

— 

January 

September 

9 

1909 

Jan.  22—29 

Oct.  25—  Nov.  1 

December 

September 

February 

October 

10 

1910 

Apr.  28—  May  5 

Oct.  8—15 

— 

April 

— 

March 

October 

10 

1911 

Apr.  11—18 

Sept.  21—28 

July 

November 

November 

May 

November 

11 

1912 

Mar.  25  —  Apr.  1 

Sept.  5-12 





— 

June 

November 

11 

1913 

Mar.  8—15 

Aug.  19—26 

February 

July 

— 

July 

December 

12 

1914 

Feb.  19—26 

Aug.  1—8 

September 

January 

August 

December 

12 

1915 

Feb.  2—9 

Nov.  3—10 

— 

February 

— 

September 

December 

12 

1916 

May  9—16 

Oct.  18—25 

April 

September 

February 

October 

— 

I 

1917 

Apr.  21—28 

Oct.  1—8 

November 



— 

November 

January 

1 

1918 

Apr.  3—10 

Sept.  14—21 

April 

March 

— 

January 

1 

1919 

Mar.  17—24 

Aug.  29—  Sept.  £ 

July 

November 

— 

January 

February 

o 

1920 

Mar.  1--8 

Aug.  12—19 

— 

— 

April 

February 

February 

2 

1921 

Feb.  12—19 

Nov.  13—20 

February 

July 



March 

March 

3 

1922 

Jan.  25—  Feb.  1 

Oct.  28—  Nov.  4 

September 



June 

April 

March 

3 

1923 

May  1—8 

Oct.  11—18 

February 

— 

May 

April 

4 

1924 

Apr.  14—  -21 

Sept.  24—  Oct.  ] 

April 

September 

August 

June 

April 

4 

1925 

Mar.  28—  Apr.  4 

Sept.  8—15 

November 

— 

— 

July 

April 

4 

1926 

Mar.  11—18 

Aug.  22—29 

^_ 

April 

October 

August 

May 

5 

1927 

Feb.  22—  Mar.  1 

Aug.  4—11 

July 

November 

— 

September 

May 

5 

1928 

Feb.  5—12 

Nov.  6—13. 





December 

October 

June 

6 

1929 

May  12—19 

Oct.  21  -28 

February 

June 

— 

December 

June 

6 

1930 

Apr.  24—  May  1 

Oct.  4—11 

September 

— 

— 

— 

June 

6 

1931 

Apr.  6—13 

Sept.  17—24 



February 

January 

January 

July 

7 

1932 

Mar.  20—27 

Sept.  1—8 

April 

September 

— 

February 

July 

7 

1933 

Mar.  4—11 

Aug.  15—22 

November 

— 

March 

March 

August 

8 

1934 

Feb.  15—22 

July  29—  Aug.  5 

April 

— 

April 

August 

8 

1935 

Jan.  28—  Feb.  4 

Oct.  31—  Nov.  7 

July 

November 

April 

May 

August 

8 

1936 

May  4—11 

Oct.  14—21 



— 

— 

June 

•September 

9 

1937 

Apr.  17—24 

Sept.  27—  Oct.  4 

February 

June 

May 

July 

September 

9 

1938 

Mar.  31—  Apr.  7 

Sept.  11—  18 

September 

— 

— 

August 

October 

10 

1939 

Mar.  14—21 

Aug.  25—  Sept.  1 

— 

February 

July 

September 

October 

10 

1940 

Feb.  25—  Mar.  3 

Aug.  7—14 

April 

September 

— 

November 

November 

11 

1941 

Feb.  8—15 

Nov.  9—16 

November 

— 

September 

December 

November 

11 

1942 

Jan.  22—29 

Oct.  24—31 



April 

— 

— 

November 

11 

1943 

Apr.  27—  May  4 

Oct.  7—14 

June 

November 

December 

January 

December 

12 

1944 

Apr.  9—16 

Sept.  20—27 





— 

February 

December 

12 

1945 

Mar.  23—30 

Sept.  4—1  1 

February 

June 

— 

March 

— 

1 

1946 

Mar.  7—14 

Aug.  18-25 

September 

— 

January 

April 

January 

1 

1947 

Feb.  18—25 

Aug.  1—8 

— 

January 

— 

May 

January 

1 

1948 

xFeb.  1—8 

Nov.  3—10 

April 

September 

February 

June 

February 

2 

1949 

May  7—14 

Oct.  17—24 

November 

_ 

— 

July 

February 

2 

1950 

Apr.  20—27 

Sept.  30—  Oct.  7 

— 

April 

March 

August 

March 

3 

THE   INDEX   TO   THE   PLANETS.  39 

Since  Mercury  and  Venus  move  in  orbits  lying  between  the  Earth  and  the  Sun,  they 
appear  from  the  Earth  to  be  always  comparatively  near  the  Sun,  and  are  visible  consequently 
only  in  the  hours  succeeding  sunset  or  preceding  sunrise.  On  the  average  they  are  most 
favourably  seen  at  the  times  when  their  angular  distance  from  the  Sun  is  greatest — the  times 
of  greatest  elongation.  But  the  circumstances  are  greatly  modified  by  the  season  of  the  year 
at  which  these  times  fall.  The  planets  move  nearly  in  the  Ecliptic.  In  the  spring,  even  in 
latitudes  as  far  north  as  England,  at  sunset  the  Ecliptic  rises  steeply  above  the  western 
horizon,  and  the  planets  are  correspondingly  high  for  a  given  elongation  from  the  Sun.  In 
autumn,  at  sunset,  the  Ecliptic  lies  low  along  the  south-western  horizon,  and  the  planets  are 
correspondingly  low. 

MERCURY. 

It  follows  from  what  has  just  been  said  that  Mercury  is  best  seen  after  sunset,  when  it 
comes  to  greatest  elongation  east  of  the  Sun  in  the  spring,  and  best  before  sunrise  when  it 
comes  to  greatest  elongation  west  in  the  autumn.  We  have  given,  therefore,  in  the  table  of 
Planetary  Phenomena  for  Mercury,  the  week  about  the  dates  of  greatest  elongations  east  and 
west,  which  come  respectively  in  spring  and  autumn ;  and  these  are  in  general  the  most 
iavourable  times  of  year  for  finding  the  planet.  Owing,  however,  to  the  considerable  inclina- 
tion, 7°,  of  the  planet's  orbit,  some  spring  and  autumn  elongations  are  much  more  favourable 
than  others,  and  exact  details  for  each  year  must  be  obtained  from  the  Nautical  Almanac. 

Mercury  moves  so  quickly  among  the  stars,  completing  a  circuit  from  conjunction  to 
conjunction  in  116  days,  that  it  is  not  possible  within  the  limits  of  this  work  to  give  an  index 
to  Mercury,  as  is  done  for  the  other  principal  planets. 


VENUS. 

The  fourth  and  fifth  columns  of  the  Table  of  Planetary  Phenomena  give  the  month  in 
which  Venus  attains  her  greatest  elongation  east  and  west  of  the  Sun.  At  these  times  she  is 
at  her  greatest  angular  distance  from  the  Sun,  and  there  is  then  on  the  average  the  greatest 
chance  of  seeing  her  well.  But  the  advantages  in  this  respect  of  different  elongations  are 
profoundly  modified  by  the  time  of  year  at  which  they  occur.  For  some  months  before 
the  time  of  greatest  eastern  elongation  Venus  is  well  to  the  east  of  the  Sun,  and  if  it  happens 
that  the  time  of  year  is  spring  when  the  Ecliptic  rises  steeply  above  the  western  horizon,  she 
will  be  a  long  way  north  as  well  as  east  of  the  Sun,  will  set  all  the  later  in  consequence,  and 
will  be  seen  under  more  favourable  conditions,  though  not  so  bright  as  when  she  reaches 
greatest  elongation,  a  month  before  she  is  most  brilliant.  Taking  everything  into  account,  in 
northern  latitudes  Venus  is  at  her  best  as  an  evening  star  when  she  comes  to  greatest  elon- 
gation east  in  June  or  July.  She  is  then  conspicuous  after  sunset  all  through  the  spring 
months,  but  is  low  and  not  so  well  seen  by  the  time  she  reaches  greatest  brightness.  If,  on 
the  other  hand,  she  comes  to  greatest  elongation  east  in  February,  and  to  greatest  brightness 
in  March,  she  makes  a  less  prolonged  but  more  splendid  appearance.  Bearing  in  mind  that 
the  factors  to  be  taken  into  account  are  (1)  elongation,  (2)  distance  north  of  the  Sun, 
(3)  brightness  (greatest  a  month  before  western  elongation),  it  will  not  be  difficult  to  frame 
similar  rules  for  her  appearances  as  a  morning  star. 


40  POPULAR   GUIDE   TO   THE   HEAVENS. 

The  Index  to  Venus,  by  which  title  we  have  designated  the  following  table,  enables  the 
approximate  position  of  the  planet  to  be  readily  ascertained  for  any  month  up  to  the  end  of 
1950.  At  the  top  of  the  index  the  names  of  the  months  are  given  in  a  horizontal  row.  The 
first  column  gives  the  year.  Corresponding  to  each  month  of  each  year  the  index  shows  a 
number,  which  is  the  number  of  one  of  the  monthly  maps,  39 — 50.  The  planet  will  be  found 
in  the  region  of  the  sky  where  the  central  meridian  of  the  map  cuts  the  "Track  of  the  Planets." 

It  must  be  remembered  that,  as  the  unit  of  time  adopted  in  this  index,  as  well  as  in  those 
of  the  other  planets,  is  a  month,  and  as  the  locality  can  only  be  indicated  by  dividing  the 
"Track  of  the  Planets,"  around  the  heavens  into  twelve  portions,  no  close  precision  need  be 
looked  for.  No  doubt  in  the  great  majority  of  cases  the  map  named  in  the  index  will  be  that 
where  the  central  meridian  lies  nearest  the  planet.  In  other  words,  the  place  of  the  planet  is 
given  to  within  an  hour.  It  may,  however,  happen  in  extreme  conditions  that  the  map 
indicated  is  not  the  best  one,  but  in  such  cases  the  right  map  always  lies  next  to  that  to  which 
the  index  refers.  Even  when  this  happens,  the  purposes  of  the  index  are  not  frustrated,  for 
the  planet  will  lie  so  near  to  the  position  in  question,  that  its  identification  will  be  unmistak- 
able, unless  on  the  rare  occasions  when  two  planets  happen  to  lie  close  together. 

EXAMPLES  TO  ILLUSTRATE  THE  USE  OF  THE  INDEX  TO  VENUS. 
Example  1. — What  will  be  the  aspect  of  Venus  in  March,  1908  1 
Solution. — The  index  given  for  March,  1908,  the  number  49.    On  this  Plate  the 
central  meridian  cuts  the  "  Track  of  the  Planets  "  in  Taurus.    This  is  the  region 
of  the  sky  required.    The  next  question  is,  when  is  this  region  above  the  horizon 
in  England  1    The  Table  at  the  top  of  Plate  49  shows  that  it  is  on  the  meridian 
about  4  p.m.  in  March.     It  will  be  well  above  the  western  horizon,  therefore,  at 
sunset,  and  Venus  will  be  splendidly  conspicuous. 

Example  2. — Where  will  Venus  be  when  brightest  as  an  evening  star  in  1943  1 
Solution. — The  Table  of  Planetary  Phenomena  shows  that  in  1943  Venus  is  at 
greatest  elongation  east  in  June.  She  will,  therefore,  be  brightest  in  July.  The 
index  gives  for  1943,  July,  the  number  40,  and  the  corresponding  chart  shows 
that  Venus  is  in  Leo.  Turning  forward  to  plates  43  and  44  we  see  that  at 
about  10  p.m.  in  July,  Venus  will  be  setting  in  the  N.W.,  and  as  the  twilight 
will  be  strong  the  planet  will  not  appear  to  great  advantage. 


(41) 
INDEX  TO  VENUS. 


A.D. 

Jan. 

Feb. 

March 

April 

May 

June 

July 

August 

Sept. 

Oct. 

Nov. 

Dec. 

1901 

44 

46 

47 

48 

49 

39 

40 

41 

42 

43 

45 

46 

1902 

46 

-  46 

46 

47 

48 

49 

50 

39 

40 

41 

43 

44 

1903 

45 

47 

48 

49 

50 

40 

41 

41 

41 

41 

41 

42 

1904 

44 

45 

46 

47 

49 

50 

39 

40 

42 

43 

44 

45 

1905 

47 

48 

48 

49 

48 

49 

49 

39 

40 

41 

42 

43 

1906 

45 

46 

47 

49 

50 

39 

40 

42 

43 

43 

43 

43 

1907 

44 

45 

4ti 

47 

48 

49 

50 

40 

41 

42 

44 

45 

1908 

46 

47 

49 

50 

50 

50 

50 

39 

40 

41 

42 

43 

1909 

44 

46 

47 

48 

49 

39 

40 

41 

42 

43 

45 

46 

1910 

46 

46 

46 

47 

48 

49 

50 

39 

40 

41 

43 

44 

1911 

45 

47 

48 

49 

50 

40 

41 

41 

41 

41 

41 

42 

1912 

44 

45 

46 

47 

49 

50 

39 

40 

42 

43 

44 

45 

1913 

47 

48 

48 

49 

48 

49 

49 

39 

40 

41 

42 

43 

1914 

45 

46 

47 

49 

50 

39 

40 

42 

43 

43 

43 

43 

1915 

44 

45 

46 

47 

48 

49 

50 

40 

41 

42 

44 

45 

1916 

46 

47 

49 

50 

50 

50 

50 

39 

40 

41 

42 

43 

1917 

44 

46 

47 

48 

49 

39 

40 

41 

42 

44 

45 

45 

1918 

45 

45 

46 

47 

48 

49 

50 

39 

40 

42 

43 

44 

1919 

46 

47 

48 

49 

50 

40 

41 

41 

41 

41 

41 

42 

1920 

44 

45 

46 

47 

49 

50 

39 

40 

42 

43 

44 

45 

1921 

47 

48 

48 

49 

48 

49 

49 

39 

40 

41 

42 

43 

1922 

45 

46 

47 

49 

50 

39 

40 

42 

43 

43 

43 

43 

1923 

44 

45 

46 

47 

48 

49 

50 

40 

41 

42 

44 

45 

1924 

46 

47 

49 

50 

50 

50 

50 

39 

40 

41 

42 

43 

1925 

44 

46 

47 

48 

49 

39 

40 

41 

42 

44 

45 

45 

1926 

45 

45 

46 

47 

48 

49 

50 

39 

40 

42 

43 

44 

1927 

46 

47 

48 

49 

50 

40 

41 

41 

41 

41 

41 

42 

19'28 

44 

45 

46 

47 

49 

50 

39 

40 

42 

43 

44 

45 

1929 

47 

48 

48 

49 

48 

49 

49 

39 

40 

41 

42 

43 

1930 

45 

46 

47 

49 

50 

39 

40 

42 

43 

43 

43 

43 

1931 

44 

45 

46 

47 

48 

49 

50 

40 

41 

42 

44 

45 

1932 

46 

47 

49 

50 

50 

50 

50 

39 

40 

41 

42 

43 

1933 

44 

46 

47 

48 

49 

39 

40 

41 

42 

44 

45 

45 

1934 

45 

45 

46 

47 

48 

49 

50 

39 

40 

42 

43 

44 

1935 

46 

47 

48 

49 

50 

40 

41 

41 

41 

41 

41 

42 

1936 

44 

45 

46 

47 

49 

50 

39 

40 

42 

43 

44 

45 

1937 

47 

48 

48 

49 

48 

49 

49 

39 

40 

41 

42 

43 

1938 

45 

46 

47 

49 

50 

39 

40 

42 

43 

43 

43 

43 

1939 

44  j  45 

46 

47 

48 

49 

50 

40 

41 

42 

44 

45 

1910 

46 

47 

49 

50 

50 

50 

50 

39 

40 

41 

42 

43 

1941 

44 

46 

47 

48 

49 

39 

40 

41 

42 

44 

45 

45 

1942 

45 

45 

46 

47 

48 

49 

50 

39 

40 

42 

43 

44 

1943 

46 

47 

48 

49 

39 

40 

40 

40 

40 

40 

41 

43 

1944 

44 

45 

46 

48 

49 

50 

39 

41 

42 

43 

44 

45 

1945 

47 

48 

48 

49 

48 

49 

49 

39 

40 

41 

42 

43 

1946 

45 

46 

47 

49 

50 

39 

40 

42 

43 

43 

43 

43 

1947 

44 

45 

46 

47 

48 

49 

50 

40 

41 

42 

44 

45 

1948 

46 

47 

49 

50 

50 

50 

50 

39 

40 

41 

42 

43 

1949 

44 

46 

47 

48 

49 

39 

40 

41 

42 

44 

45    45 

1950 

45 

45 

46 

47 

48 

49 

50 

39 

40 

42 

43 

44 

42  POPULAR   GUIDE   TO   THE   HEAVENS. 


MARS. 

We  have  already  explained,  in  connection  with  Plate  6,  the  phases  of  Mars  and  the 
effect  of  the  eccentricity  of  his  orbit,  whereby  the  distance  of  the  planet,  and  its  consequent 
apparent  diameter,  is  dependent  upon  the  time  of  year  at  which  opposition  occurs — the  most 
favourable  date  being  the  end  of  August,  and  the  least  favourable  the  end  of  February.  It 
should  not  be  forgotten,  however,  that  in  countries  as  far  north  as  England,  Mars  at  an 
August  opposition  is  very  low  down  in  the  south,  and  unfavourably  placed  for  telescopic 
scrutiny.  It  may  well  happen  that  at  an  opposition  occuring  later  in  the  year,  say  in  October, 
the  planetary  detail  is  better  seen,  because  of  the  greater  elevation  at  which  the  planet  crosses 
the  meridian,  although  the  planet  is  farther  away  and  apparently  smaller. 

The  conditions  for  a  favourable  view  of  Mars  and  other  exterior  planets  are  much  simpler 
than  those  for  Venus  and  Mercury.  The  planet  is  best  seen  when  it  is  in  opposition  to  the 
Sun  ;  it  is  then  on  the  meridian  at  midnight,  and  is  up  for  practically  the  whole  of  the  night. 

The  table  of  planetary  phenomena  gives  the  dates  of  the  opposition  of  Mars,  and  we  can 
judge  from  them  at  once  how  favourable  the  opposition  will  be.  The  Index  to  Mars  enables 
us  to  find  in  what  region  of  the  sky  the  planet  will  be  at  any  time. 

EXAMPLES  TO  ILLUSTRATE  THE  USE  OF  THE  INDEX  TO  MARS. 

Example  1. — Find  the  place  of  Mars  in  August,  1907. 

Solution. — The  Index  gives  the  reference  45,  and  the  corresponding  chart  shows 
that  the  planet  lies  between  Capricornus  and  Sagittarius.  It  is  on  the 
meridian  about  10  p.m.,  but  is  considerably  south.  A  reference  to  the  Table  of 
Planetary  Phenomena  shows  that  Opposition  occurred  in  June,  which  is  not  a 
very  favourable  case,  and,  since  the  planet  is  also  far  south,  it  will  not  be  very 
favourably  placed  for  observation  in  England  in  1907,  August. 

Example  2. — When  is  Mars  on  the  meridian  in  April,  1938  ? 

Solution. — The  index  gives  48,  and  the  corresponding  map  shows  that  Mars  is  in 
Aries.  Reference  to  the  table  in  the  top  corner  shows  that  Aries  is  on  the 
meridian  at  noon  in  April,  during  daylight ;  the  planet  will,  therefore,  not  be 
visible. 


(43) 
INDEX  TO   MARS. 


A.D. 

Jan. 

Feb. 

March 

April 

May 

June 

July 

August 

Sept. 

Oct. 

Nov. 

Dec. 

1901 

41 

40 

40 

40 

40 

41 

41 

42 

43 

43 

44 

45 

1902 

40 

46 

47 

48 

49 

49 

50 

39 

40 

40 

41 

41 

1903 

42 

42 

41 

41 

41 

41 

42 

4-2 

43 

44 

44 

45 

1904 

46 

47 

47 

48 

49 

50 

50 

39 

40 

40 

41 

42 

1905 

42 

42 

43 

43 

43 

43 

43 

43 

44 

44 

45 

46 

1906 

47 

47 

48 

49 

50 

50 

39 

40 

40 

41 

42 

42 

1907 

43 

44 

44 

45 

45 

44 

45 

45 

45 

46 

46 

47 

1908 

48 

48 

49 

50 

50 

39 

40 

40 

41 

42 

42 

43 

1909 

44 

45 

45 

46 

47 

47 

47 

47 

47 

47 

47 

47 

1910 

48 

48 

49 

50 

50 

39 

40 

40 

41 

42 

43 

43 

1911 

44 

44 

45 

46 

47 

47 

48 

49 

49 

49 

49 

49 

1912 

49 

49 

50 

50 

39 

40 

40 

41 

41 

42 

43 

43 

1913 

44 

45 

46 

47 

47 

48 

49 

49 

50 

39 

39 

39 

1914 

39 

39 

39 

39 

40 

40 

41 

42 

42 

43 

44 

44 

1915 

45 

46 

47 

48 

48 

49 

50 

39 

39 

40 

40 

41 

1916 

41 

40 

40 

40 

40 

41 

41 

42 

43 

43 

44 

45 

1917 

46 

46 

47 

48 

49 

49 

50 

39 

40 

40 

41 

41 

1918 

42 

42 

41 

41 

41 

41 

42 

42 

43 

44 

44 

45 

1919 

46 

47 

47 

48 

49 

50 

50 

39 

40 

40 

41 

42 

1920 

42 

42 

43 

42 

42 

42 

42 

43 

43 

44 

45 

46 

1921 

47 

47 

48 

49 

49 

50 

39 

40 

40 

41 

41 

42 

1922 

43 

43 

44 

44 

44 

44 

44 

44 

44 

45 

46 

46 

1923 

47 

48 

48 

49 

50 

39 

39 

40 

41 

41 

42 

42 

1924 

43 

44 

45 

45 

46 

46 

46 

46 

46 

46 

46 

47 

1925 

47 

48 

49 

49 

50 

39 

39 

40 

41 

41 

42 

43 

1926 

43 

44 

45 

46 

46 

47 

48 

48 

48 

48 

48 

48 

1927 

48 

49 

49 

50 

39 

39 

40 

41 

.41 

42 

43 

43 

1928 

44 

45 

45 

46 

47 

48 

48 

49 

50 

50 

50 

50 

1929 

50 

50 

50 

39 

39 

40 

41 

41 

42 

42 

43 

44 

1930 

45 

46 

46 

47 

48 

49 

49 

50 

39 

39 

40- 

40 

1931 

39 

39 

39 

39 

40 

40 

41 

42 

42 

43 

44 

44 

1932 

45 

46 

47 

48 

48 

49 

50 

39 

39 

40 

40 

41 

1933 

41 

41 

41 

41 

41 

41 

42 

42 

43 

44 

44 

45 

1934 

46 

47 

47 

48 

49 

50 

50 

39 

40 

40 

41 

42 

1935 

42 

42 

43 

42 

42 

42 

42 

43 

43 

44 

45 

46 

1936 

47 

47 

48 

49 

49 

50 

39 

40 

40 

41 

41 

42 

1937 

43 

43 

44 

44 

43 

43 

43 

43 

44 

44 

45 

46 

193S 

47 

47 

48 

49 

50 

50 

39 

40 

40 

41 

42 

42 

1939 

43 

44 

44 

45 

45 

45 

45 

45 

45 

45 

46 

46 

1940 

47 

48 

48 

49 

50 

50 

39 

40 

40 

41 

42 

42 

1941 

43 

44 

45 

45 

46 

47 

47 

47 

47 

47 

47 

47 

1942 

48 

48 

49 

50 

50 

39 

40 

40 

41 

42 

42 

43 

1943 

44 

44 

45 

46 

47 

47 

48 

49 

49 

50 

50 

50 

1944 

50 

50 

50 

39 

39 

40 

41 

41 

42 

42 

43 

44 

1945 

45 

46 

46 

47 

48 

49 

49 

50 

39 

39 

40 

40 

1946 

39 

39 

39 

39 

40 

40 

41 

42 

42 

43 

44 

44 

1947 

45 

46 

47 

48 

48 

49 

50 

39 

39 

40 

40 

41 

194S 

41 

40 

40 

40 

40 

41 

41 

42 

43 

43 

44 

45 

1949 

46 

46 

47 

48 

49 

49 

50 

39 

40 

40 

41 

41 

1950 

42 

42 

41 

41 

41 

41 

42 

42 

43 

44 

44 

45 

44  POPULAR   GUIDE   TO   THE   HEAVENS. 


JUPITER. 

Since  the  eccentricity  of  Jupiter's  orbit  is  not  large,  and  it  lies  far  outside  the  orbit  of 
the  Earth,  successive  oppositions  of  the  planet,  every  thirteen  months,  would  be  almost 
equally  favourable,  were  it  not  for  the  fact  that  at  a  summer  opposition  the  planet  is  always 
low  in  England,  whereas  at  a  winter  opposition  it  is  high.  The  dates  of  opposition  from 
1901— 1950  are  found  in  the  Table  of  Planetary  Phenomena.  The  position  of  the  planet  in 
the  sky  at  any  time  can  be  found  as  usual  from  the  following  index. 

EXAMPLES  TO  ILLUSTRATE  THE  USE  OF  THE  INDEX  TO  JUPITER. 
Example  I. — In  what  part  of  the  heavens  will  Jupiter  be  in  January,  1910 1 

Solution. — The  index  to  Jupiter  gives  for  this  month  the  number  41,  and  the 

corresponding  Plate  shows  that  the  planet  is  between  Leo  and  Virgo,  and 

consequently  close  to  the  equator.     The  Table  at  the  top  shows  that  this 

region  is  on  the  meridian  about  4  a.m.  in  January.     The  planet  will  therefore 

.    rise  about  10  p.m.,  and  will  be  well  visible  after  midnight. 

Example  2. — In  July,  1919,  a  bright  planet  is  seen  in  the  west  at  sunset.  Is  it 
Jupiter  ? 

Solution. — The  index  to  Jupiter  gives  for  July,  1919,  the  reference  to  chart  40, 
which  shows  that  Jupiter  is  a  little  to  the  west  of  the  Sickle  in  Leo.  Turning  on 
to  Plates  43  and  44,  we  see  that  the  Sickle  is  low  down  at  8  p.m.,  and  is  on  the 
horizon  at  10  p.m.  Jupiter  will  therefore  be  scarcely  visible  at  Sunset,  and  the 
planet  in  question  is  probably  not  Jupiter.  Reference  to  the  Index  to  Venus 
shows  that  the  planet  higher  up  in  the  west  at  that  time  is  Venus. 


(45) 
INDEX  TO  JUPITER. 


A.D. 

Jan. 

Feb. 

March 

April 

May 

June 

July 

August 

Sept. 

Oct. 

Nov. 

Dec. 

1901 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

45 

1902 

45 

45 

45 

45 

45 

45 

45 

46 

46 

46 

46 

46 

1903 

40 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

1904 

47 

47 

48 

48 

48 

48 

48 

48 

48 

48 

48 

48 

1905 

48 

48 

48 

49 

49 

49 

49 

49 

49 

49 

49 

49 

1906 

49 

49 

49 

49 

50 

50 

50 

50 

50 

50 

50 

50 

1907 

50 

50 

50 

50 

50 

39 

39 

39 

39 

39 

39 

39 

1908 

39 

39 

89 

39 

39 

39 

40 

40 

40 

40 

40 

40 

190!) 

40 

40 

40 

40 

40 

40 

40 

41 

41 

41 

41 

41 

1910 

41 

41 

41 

41 

41 

41 

41 

41 

42 

42 

42 

42 

1911 

42 

42 

42 

42 

43 

43 

43 

43 

43 

43 

44 

44 

1912 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

45 

1913 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

'  1914 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

1915 

46 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

1916 

47 

47 

48 

48 

48 

48 

48 

48 

48 

48 

48 

48 

1917 

48 

48 

48 

49 

49 

49 

49 

49 

49 

49 

49 

49 

1918 

49 

49 

49 

49 

50 

50 

50 

50 

50 

50 

50 

50 

1919 

39 

39 

39 

39 

39 

39 

40 

40 

40 

40 

40 

40 

1920 

40 

40 

40 

40 

40 

40 

40 

41 

41 

41 

41 

41 

1921 

41 

41 

41 

41 

41 

41 

41 

41 

42 

42 

42 

42 

1922 

42 

42 

42 

42 

42 

42 

42 

42 

42 

43 

43 

43 

1923 

43 

43 

43 

43 

43 

43 

43 

43 

43 

43 

44 

44 

1924 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

45 

1925 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

1926 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

1927 

46 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

1928 

47 

47 

48 

48 

48 

48 

48 

48 

48 

48 

48 

48 

1929 

48 

48 

48 

49 

49 

49 

49 

49 

49 

49 

49 

50 

1930 

50 

50 

50 

50 

50 

39 

39 

39 

39 

39 

39 

39 

1931 

39 

39 

39 

39 

39 

39 

40 

40 

40 

40 

40 

40 

1932 

40 

40 

40 

40 

40 

40 

40 

41 

41 

41 

41 

41 

1933 

41 

41 

41 

41 

41 

41 

41 

41 

42 

42 

42 

42 

1934 

42 

42 

42 

42 

42 

42 

42 

42 

42 

43 

43 

43 

1935 

43 

43 

43 

43 

43 

43 

43 

43 

43 

43 

44 

44 

1936 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

45 

1937 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

46 

1938 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

1939 

46 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

1940 

47 

47 

48 

48 

48 

48 

48 

48 

48 

48 

49 

49 

1941 

49 

49 

49 

49 

50 

50 

50 

50 

50 

50 

50 

50 

1942 

50 

50 

50 

50 

50 

39 

39 

39 

39 

39 

39 

39 

1943 

39 

39 

39 

39 

39 

39 

40 

40 

40 

40 

40 

40 

1944 

40 

40 

40 

40 

40 

40 

40 

41 

41 

41 

41 

41 

1945 

41 

41 

41 

41 

41 

41 

41 

41 

42 

42 

42 

42 

1946 

42 

42 

42 

42 

42 

42 

42 

42 

42 

43 

43 

43 

1947 

43 

43 

43 

43 

43 

43 

43 

43 

43 

43 

44 

44 

1948 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

45 

1949 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

1950 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46  POPULAR   GUIDE   TO   THE  HEAVENS. 


SATURN. 

What  we  have  said  of  Jupiter  applies  equally  well  to  Saturn.  An  opposition  is 
favourable  or  unfavourable  according  as  the  planet  is  in  the  northern  or  southern  part  of  the 
ecliptic.  But  there  is  the  further  consideration,  that  in  the  telescope  a  great  deal,  and  to 
the  naked  eye  something,  depends  upon  whether  the  rings  are  seen  well  open  or  closed  up. 
When  the  rings  are  fully  open  the  planet  is  nearly  a  magnitude  brighter  than  when  they  are 
edgewise.  For  the  appearance  of  the  rings  at  any  time  reference  may  be  made  from  the  last 
column  of  the  Table  of  Planetary  Phenomena  to  Plate  6. 

EXAMPLES  TO  ILLUSTRATE  THE  USE  OF  THE  INDEX  TO  SATURN. 
Example  1. — When  will  Saturn  be  found  near  the  Pleiades  ? 

Solution. — The  Pleiades  are  close  to  the  central  meridian  of  Chart  49.  The  index 
gives  the  reference  number,  49,  for  Saturn  from  May,  1911  to  May,  1913. 
During  these  two  years  the  planet  will  pass  from  west  to  east  across  the  section 
of  the  "track  of  the  planets"  central  on  Chart  49,  and  will  be  close  to  the  Pleiades 
in  the  summer  of  1912.  The  last  column  of  the  Table  of  Planetary  Phenomena 
gives  the  number  11,  and  reference  to  Plate  6  shows  that  the  rings  are  nearly  at 
their  widest  open.  A  repetition  of  the  circumstance  occurs  in  1941  and  1942. 

Example  2. — When  will  Saturn  be  found  near  the  meridian  at  10  p.m.  in  February  ? 

Solution. — Eefereuce  to  the  monthly  maps  shows  that  the  region  of  the  sky  which 
comes  to  the  meridian  at  10  p.m.  in  February  is  on  the  meridian  at  midnight  in 
January,  and  the  Index  number  is  39.  We  therefore  look  for  this  number  in 
the  Index  to  Saturn  in  the  column  for  February,  and  find  the  years  1917,  1918, 
1946,  1947.  On  these  years  the  planet  will  be  on  the  meridian  about  10  p.m. 
in  February. 


(47) 
INDEX  TO  SATURN. 


A.D. 

Jan. 

Feb. 

March 

April 

May 

June 

July 

August 

Sept. 

Oct. 

Nov. 

Dec. 

1901 

4o 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

1902 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

1903 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

mot 

45 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

1905 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

1906 

46 

46 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

1907 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

1908 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

1909 

47 

47 

47 

48 

48 

48 

48 

48 

48 

48 

48 

48 

1910 

48 

48 

48 

48 

48 

48 

48 

48 

48 

48 

48 

48 

1911 

48 

48 

48 

48 

49 

49 

49 

49 

49 

49 

49 

49 

1912 

49 

49 

49 

49 

49 

49 

49 

49 

49 

49 

49 

49 

1913 

49 

49 

49 

49 

49 

50 

50 

50 

50 

50 

50 

50 

1914 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50 

1915 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50 

1916 

50 

50 

50 

50 

50 

50 

39 

39 

39 

39 

39 

39 

1917 

39 

39 

39 

39 

39 

39 

39 

39 

39 

39 

39 

39 

1918 

39 

39 

39 

39 

39 

39 

39 

40 

40 

40 

40 

40 

1919 

40 

40 

40 

40 

40 

40 

40 

40 

40 

40 

40 

40 

1920 

40 

40 

40 

40 

40 

40 

40 

40 

41 

41 

41 

41 

1921 

41 

41 

41 

41 

41 

41 

41 

41 

41 

41 

41 

41 

1922 

41 

41 

41 

41 

41 

41 

41 

41 

41 

42 

42 

42 

1923 

42 

42 

42 

42 

42 

42 

42 

42 

42 

42 

42 

42 

1924 

42 

42 

42 

42 

42 

42 

42 

42 

42 

42 

42 

42 

1925 

42 

42 

42 

42 

42 

42 

42 

42 

42 

42 

43 

43 

1926 

43 

43 

43 

43 

43 

43 

43 

43 

43 

43 

43 

43 

1927 

43 

43 

43 

43 

43 

43 

43 

43 

43 

43 

43 

44 

1928 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

1929 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

1930 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

44 

1931 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

1932 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

45 

1933 

45 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

1934 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

1935 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

46 

1936 

46 

46 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

1937 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

47 

1938 

47 

47 

47 

48 

48 

48 

48 

48 

48 

48 

48 

48 

1939 

48 

48 

48 

48 

48 

48 

48 

48 

48 

48 

48 

48 

1940 

48 

48 

48 

48 

49 

49 

49 

49 

49 

49 

49 

49 

1941 

49 

49 

49 

49 

49 

49 

49 

49 

49 

49 

49 

49 

1942 

49 

49 

49 

49 

49 

49 

49 

49 

49 

49 

49 

49 

1943 

49 

49 

49 

49 

49 

50 

50 

50 

50 

50 

50 

50 

1944 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50 

1945 

50 

50 

50 

50 

50 

50 

39 

39 

39 

39 

39 

39 

1946 

39 

39 

39 

39 

39 

39 

39 

39 

39 

39 

39 

39 

1947 

39 

39 

39 

39 

39 

39 

39 

40 

40 

40 

40 

40 

1948 

40 

40 

40 

40 

40 

40 

40 

40 

40 

40 

40 

40 

1949 

40 

40 

40 

40 

40 

40 

40 

40 

41 

41 

41 

41 

1950 

41 

41 

41 

41 

41 

41 

41 

41 

41 

41 

41 

41 

48  POPULAR  GUIDE   TO   THE  HEAVENS. 


THE  NAMING  OF  AN  UNKOWN  PLANET. 

The  beginner  will  sometimes  notice  a  bright  star-like  object  which  he  knows  is  not  a 
Star,  for  it  is  not  represented  on  the  maps.  He  infers  that  it  must  be  a  planet,  and  he  desires 
to  find  its  name. 

It  may  be  assumed  that  the  object  must  be  one  of  the  four  bodies— Venus,  Mars,  Jupiter, 
or  Saturn.  With  the  aid  of  the  Planetary  Index  it  is  a  simple  matter  to  determine  which  of 
the  four  the  unknown  object  must  be. 

We  will  illustrate  the  process  by  two  examples  : — 

Example  1. — In  May,  1905,  a  planet  is  seen  in  the  south  at  midnight.  It  is 
certainly,  therefore,  not  Venus.  Plate  43  shows  that  it  is  in  Scorpio,  and  its 
index  number  is  43.  The  index  numbers  in  May,  1905,  for  Mars,  Jupiter,  and 
Saturn  are  respectively,  43,  49,  46.  The  planet  is  therefore  Mars.  This  might 
also  be  inferred  from  the  Table  of  Planetory  Phenomena.  Since  the  planet  is 
near  the  meridian  at  midnight  it  is  close  to  opposition  ;  and  in  1905  the  table 
shows  that  Jupiter  is  in  opposition  in  November,  Saturn  in  August,  and  Mars 
in  May. 

Example  2. — In  April,  1916,  two  planets  will  be  seen  at  8  p.m.,  not  far  from  one 
another,  fairly  well  up  in  the  west,  and  a  third  is  in  the  south.  Name  the 
planets.  Chart  40  gives  the  aspect  of  the  sky  at  8  p.m.  in  April,  and  the 
planets  in  the  west  will  be  in  Taurus  or  Gemini ;  that  in  the  south  will  be  in 
Leo,  and  its  index  number  will  be  40,  the  number  of  the  chart.  To  find  the 
index  numbers  of  the  others  we  must  turn  to  the  chart  which  shows  the  region 
between  Taurus  and  Gemini  on  the  central  meridian,  that  is,  to  Chart  50.  The 
index  numbers  of  the  two  planets  in  the  west  will  probably  both  be  50,  but  may 
possibly  be  49  or  39.  The  indexes  for  April,  1916,  give  for  Venus  50,  for  Mars 
40,  for  Jupiter  48,  and  for  Saturn  50.  The  planet  in  the  south  is  therefore 
Mars.  In  the  west  are  Saturn  and  Venus ;  the  latter  will  be  the  brighter  of 
the  two. 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS 


Plate  3 9 


January    Midnight 

itiul  also  as 


Jan.?  Midnight 

Fel?  JO.  P.M. 

Mc^k  S.  P.M. 

April  6.  P  MfDti^iflfo) 

May  4.  P.    M       " 

June  2.  P  M. 


Sou  ill 
or  at  Sidei'ealTime     7^  57 


OF 

ORNl 


BALL'SPOPUUARGUIDE  TO  THE  HEAVENS 


Plate  40 


February  Midnight 
•f  o 

ajid  also  as  follows 


Jan?  2.  A.M. 
Feb?  Midnight 
March  lO.p'.M. 
April  8.  P.  M. 
May  6.  P. 
Jwie  4.  PN. 


July    2,  P2£(Da0itf*t 
Auyt       Noon 
Septr    10.  A.M.       " 
Oetr      8.  A.M.     " 

M. 

M. 


6.  A. 


•l.A. 


South 
or   at  Sidereal  Time    9h  37  m 

Z  3  4 




ffeorye  Pruti*  A  S,j 


BALL'S  POPULAR  GUIDE  TO  THEHEAVENS 


Plate  41 


Marr.li     Midnight 

curd  also  as  follows 


Jan.y  4.  AM. 
feb?  2.  A.M.. 
March  Mi,liii,ilil 
April  10.  P  M 
May  S.P  K 
June  6. 


July   4.  P.M.  (Davliriht) 
2.  P.M.      '  • 


South 
or  at  Sidereal  Time  II11  37m 


George  FhjUp  &  Son 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS 


Plate  42 


April     Midnight 

and  also  as   folio n-s 


Jan?     6.  A.M. 
Fcb?     4.  A.M. 

Xai-ch    2.  A.M. 


July    0.  P.  MjDajhght, 
Aug*     4.PM. 

Sept?  2.  P.  M 


South 
or  at  Sidereal  Time  B1.1  3?m 


George.  Philip  <fe  Son 


'VERS 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS 


Plate  43 


J, i.n-7  a,  A.M. 

Pel?  6.  A.M. 

M,n;-h  4.  A.M. 

April  2. A.M. 

Ma-y  Midnight 

June.  10.  P.  K. 


May  Midnight 

<m<l  (i  I  so    us    follow* 


.  Inly  8 
dug!  6.  P.  11 
SepT  4.PM 
2  PM. 

>n, 


South 
or  at  Sidereal  Time 


234 


The  Lond^JT'i.  Geapraphi&il  Irisa 


OF  THE 

"NIVERSITY 


BALL'S  POPULAR  GUIDE  TO  THEHEAVENS 


Plate  44 


Jan?'  10.  A.  M  (Daylight] 

F&b?  a   A.M. 

M.u-clv  6.  A.M. 

April  4-,  A.M. 
May  2,  A.M. 

Jwr^e. 


June  Midnight 
and  also  as   follows 


July     10.   P.M. 

Aiy!    8  .  P.  M. 
Sf.ptT  6.     P 

oar   4.   P.  M. 

2.    P.   M 


South 
or   at  Sidereal  Time   17>  37m 


"  • 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS 


Plate  45 


Jan  rv      Noon  (tttLylight) 
/>•/,:'      10.  A.M. 
M,i,:-h  8.  A.M. 
A/>ril     6  AM 

M, 

Jl 


July   Midnight 
and   a,2.s0    /«.<  fttunf» 


4.  A.M. 


2. 


July     MuJniijht 
Septr  8.  P.  M. 

Oct."  6.  P.M. 

JVov!'  4  P.   M    fDayliffht, 

Dec.!    2PM. 


South 
or  at  Siclei-ealTime  19h37m 


2  3  4 


UN' 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS 


Plate  4 6 


Jan?  2.   P.M.fDaylif/lit; 

Fehr  N&Ofl* 

March.  10,  AM.          " 
April 

May 


August  Midnight 

and,    also    us    follows 


July     2.  A.M. 
?  Midniaht 

^ 


South 
or  at  Sidevejaiime  2 


' '          **"  '  -—  *r    t>   ^ 

f    .         or  - 

(UNIVERSITY 

\  OF 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS 


PI  ate  47 


September  Midnight 

also   a.s    follows 


J:tn  .-'  4.  P  M/ 

FeV  2.  P.  M. 

Murch  Nofin 

April  10.A..M 

Xnv  8.  A  M. 

Jun<-  H.  A.  If 


Sozith 
or  at  SidercalTime 


orge  Philip 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS 


Plate  48 


Jan?    6,   P.  M 

Fcl?     4.    P  M.(DayKyhbl 

Xarah  -?,    P  M. 

April      \',>,m 
Xa.ij       W.   A.M. 
Ju.ne       8.    A .  M. 


October  Mi 

o 

an  J.  also    fis  fodowy 


July       6,  A 

Aug?  4,  A. ,!/  . 
Septr  2. A.M. 
OctT  Midrdaht 

Nm.r  LO.P.'M. 

M  . 


South 
or  at  SiderealTime'l11  371!1 


The-  IioruiaTi,  GeoffrapJiLcaL 


OP  THE 

UNIVERSITY 

OF 


BALL'S  POPULAR  GUIDE  TO  THE  HEAVENS 


Plate  49 


Jan?     8.  P.M. 
Frb?     6.  P.M. 
Ma.r-.-Ji  4.  P 
April  Z.P   M. 
Ma.\     Niton 
June   10.  A.M. 


November  Midniplit 

o 

O7i.cZ     also     CLS    folltrws 


JWy       8,  A    M. (Daylight  I 
Autjt      6,  A.M. 
Sep'tr   4.  A.M. 
2,  A.M. 
yht 


South 
or  at  Sidereal  Time 


LoncUm,  GeoffraphLcal  In,sta#u 


BALL'S  POPULAR  GUIDE  TOTHEHEAVENS 


Plate  50 


December  Midnight 

mul  ditto   us  Follows 


South 

or   at   SiderealTime    51.1    37m 


2  a  <; 


Geerge  Philip  & 


rhe-  London.  GeagraphLcaL 


(49) 


CHAPTER    VII.— TEE  STAR  MAPS. 

PLATES  51  TO  70. 

The  student  who  has  made  himself  familiar  with  the  appearance  and  movements  of  the 
Constellations,  and  has  acquired  a  facility  in  identifying  the  brighter  Stars,  will  soon  feel  the 
need  of  something  further.  More  especially  will  this  be  the  case  if  he  has  the  use  of  a 
telescope  of  even  moderate  dimensions ;  and  it  is  to  meet  these  requirements  that  the  Star 
Maps  on  Plates  51  to  70  have  been  prepared 

The  first  step  in  drawing  a  map  is  to  decide  on  the  nature  of  the  projection  to  be 
employed.  It  must  be  understood  that  no  flat  maps  can  give  a  perfectly  faithful  representation 
of  a  curved  surface,  and  whatever  method  of  projection  is  resorted  to,  the  result  must  represent 
the  surface  in  a  more  or  less  distorted  form.  The  Stars  appear  to  be  situated  on  the  surface 
of  a  sphere,  and  however  we  may  attempt  to  depict  them,  we  cannot  include  any  large  portion 
of  the  sphere  exactly  as  it  appears  to  the  eye.  The  form  of  projection  which  I  have  used 
in  these  maps  is  that  known  as  the  conical  projection,  and  in  adopting  it  I  follow  Argelande; 
who  employed  this  method  in  his  great  Durchmusterung  Atlas,  which  represents  more  th;m 
300,000  Stars  in  the  Northern  Hemisphere  alone. 

Imagine  two  cones  touching  the  sphere  around  the 
circle  of  45°  declination,  north  and  south.  These  are 
intersected  by  tangent  planes  at  the  Poles,  and  by  a 
cylinder  touching  the  sphere  around  the  Equator;  see  ad- 
joining figure.  Each  star  on  the  sphere  is  joined  to  the 
centre,  and  the  joining  line  when  produced  necessarily 
cuts  some  one  of  the  enveloping  surfaces  in  a  point  which 
is  the  projection  of  the  star.  The  equatorial  girdle  and 
the  two  cones  are  each  divided  into  six  equal  parts, 
which  admit  of  being  laid  out  flat ;  and  the  eighteen 
parts  thus  obtained,  together  with  the  two  polar  planes, 
make  up  the  twenty  maps  which  represent  the  entire 
sphere. 

The  top  and  bottom  margins  of  each  of  these  maps, 
with  the  exception  of  the  first  and  last,  are  divided  so 
as  to  read  Right  Ascensions. 

Only  the  hour  lines  have  been  drawn  on  the  maps, 
so  as  to  avoid  overcrowding,  and  for  the  same  reason 
only  the  circles  corresponding  to  every  tenth  degree  in 

Declination  have  been  given.  But  by  the  aid  of  the  divisions  around  the  margin,  it  is  easy  to 
read  the  position  of  a  star,  or  to  enter  any  desired  object  with  all  requisite  accuracy.  For  this 
purpose  it  will  be  found  convenient  to  copy  the  scale  in  Declination,  which  is  given  on  the 
margin  of  each  map,  on  the  edge  of  a  sheet  of  paper.  If,  then,  it  is  desired  to  enter  on  the  map 
the  position  of  any  object  (say  a  comet)  whose  R.  A.  and  Declination  are  known,  it  is  only  neces- 
sary to  set  this  sheet  of  paper  so  that  the  graduated  edge  cuts  the  top  and  bottom  circles  at 


50  POPULAR  GUIDE  TO   THE   HEAVENS. 

the  R.A.  of  the  object,  and  to  put  a  dot  on  the  map  at  the  point  of  the  scale  corresponding  to 
its  Decimation.  In  the  same  way  the  position  of  any  object  entered  on  the  map  may  be  read 
off.  In  the  case  of  maps  51  and  70,  the  method  of  reading  off  positions  is  somewhat  different 
In  these  the  declination  scale  will  be  found  on  the  radius  corresponding  to  Oh-,  6h-,  12h-  or  24h. 
This  scale  should  be  rotated  around  the  centre  until  it  passes  through  the  star  whose  position 
is  required.  The  R.A.  will  then  be  found  at  the  point  of  the  circumference  where  the  scale 
cuts  it,  and  the  Declination  will  be  read  from  the  scale  itself.  The  epoch  for  which  the  places 
are  given  is  1880. 

It  has  been  arranged  that  each  zone  of  maps  overlaps  those  north  and  south  of  it  for  a 
distance  of  five  degrees  in  Declination,  and  each  map  of  a  zone  overlaps  those  preceding  and 
following  it  for  a  space  of  40  mins.  in  R.A. 

In  order  to  avoid  breaking  up  conspicuous  star  groups,  I  have  made  the  zero,  from  which 
the  hour  circles  are  measured,  pass  through  the  centre  of  the  first  map  (No.  52)  of  the  inter- 
mediate zones,  while  the  same  circle  divides  the  first  and  last  maps  of  the  equatorial  zone- 
By  this  mode  of  dividing  the  heavens  it  has  been  found  possible  to  comprise  each  of  the 
more  striking  configurations  of  stars  within  a  single  map.  The  only  exception  is  the  great 
square  of  Pegasus,  which  will  be  found  on  Plates  52,  58,  and  63.  For  convenience  in  passing 
from  one  map  to  another,  the  numbers  of  the  plates  which  represent  adjacent  portions  of  the 
sky  have  been  printed  just  inside  the  margin. 

In  the  construction  of  these  maps  I  have  followed,  to  a  great  extent,  the  Uranometrie 
Generate  of  Houzeau.  It  contains  all  the  stars  visible  to  the  naked  eye  under  the  most  favour- 
able circumstances,  and  the  number  amounts  to  nearly  6,000. 

In  the  nomenclature  of  the  stars,  however,  I  have  departed  considerably  from  Houzeau, 
doing  away  in  general  with  letters  (other  than  those  of  the  Greek  alphabet),  and  substituting, 
wherever  possible,  Flamsteed's  numbers. 

I  have  followed  Houzeau  throughout  in  the  estimation  of  star  magnitudes,  as  by  so  doing 
I  obtained  a  uniform  system  over  the  whole  sky,  both  in  the  Northern  and  Southern  Hemi- 
spheres, determined  by  a  single  observer  in  the  same  climate  and  within  a  short  time.  His 
work,  besides,  is  more  recent  than  that  of  Argelander,  Heis,  or  Behrmann.  I  have  further,  for 
simplicity,  limited  the  number  of  magnitudes  given  by  Houzeau  to  six,  namely,  1,  2,  3,  4,  5, 
and  6,  which  will  be  found  sufficient  for  all  ordinary  purposes.  These  I  have  indicated  on  the 
maps  themselves,  as  shown  by  the  scale  at  the  foot  of  each  map,  where,  in  addition  to  the  size 
of  the  dot  representing  the  star,  its  magnitude  is  denoted  by  the  number  of  rays  diverging  from 
it.  Thus  all  stars  of  the  first  magnitude  possess  6  rays,  those  of  the  second  magnitude  5  rays, 
and  so  on.  The  magnitude  of  a  star  may  be  found  by  subtracting  the  number  of  the  rays 
from  seven,  except  for  the  sixth  magnitude,  in  which  case  the  single  ray  has  been  omitted,  stars 
of  this  class  being  represented  by  a  simple  dot. 

Throughout  the  maps  a  large  number  of  the  stars  will  be  found  accompanied  by  the 
letter  D.  This  signifies  that  the  star,  though  appearing  as  a  single  body  to  the  naked  eye, 
is  in  reality  double.  This  does  not  however  denote  necessarily  any  physical  connection 
between  the  two  stars,  but  merely  that  the  point  of  light  thus  characterized  will  be 
found  to  break  up  into  two  with  the  aid  of  a  small  telescope.  In  many  cases  there  is 
a  real  connection  ;  the  two  stars  form  a  binary  pair,  and  revolve  round  their  common 
centre  of  gravity  under  the  action  of  their  mutual  gravitation  ;  a  number  of  the  most 
interesting  cases  will  be  found  in  the  list  of  select  telescopic  objects.  In  other  cases, 
the  connection  is  merely  apparent ;  the  stars  happen  to  be  nearly  in  the  same  direction  as 
seen  from  the  solar  system ;  but  they  are  probably  at  very  different  distances  from  us. 


THE  STAR  MAPS. 


51 


VARIABLE  STARS. 

A  number  of  stars  will  be  found  in  the  maps  marked  (Var.),  which  indicates  that 
their  light  is  not  constant,  but  is  subject  to  fluctuations,  in  some  cases  perfectly  regular, 
so  that  their  times  of  greatest  brightness  can  be  predicted  to  the  minute  ;  in  other  cases, 
more  or  less  regular,  so  that  their  maxima  can  be  predicted  to  within  a  week  or  a  month ; 
in  yet  other  cases,  so  irregular  that  no  law  has  yet  been  found  for  their  variation.  In 
the  following  tables  I  have  given  those  variable  stars  which  are  plainly  visible  to  the 
naked  eye  at  least  at  their  maximum  brightness.  They  may  be  easily  identified  from 
the  star  maps.  There  are  many  hundreds  of  variable  stars  which  never  rise  to  naked 
eye  brilliancy,  but  for  their  identification  means  more  elaborate  than  those  of  the  present 
work  are  required.  Without  detailed  charts  of  the  fainter  stars  it  is  very  hard  to  find 
them.  For  information  regarding  them  the  student  is  referred  to  the  lists  published  in 
the  Companion  to  the  Observatory;  for  their  identification  one  needs  the  star  maps  of 
Argelander's  Durchmusterung,  and  of  the  Cordoba  Durchmusterung,  or,  better  still,  the 
Atlas  of  Variable  Stars,  published  by  Father  Hagen,  of  Georgetown,  D.C. 

"ALGOL"  VARIABLES. 

There  is  a  small  class  of  stars  whose  type  of  variation  is  characteristic,  and  of  which 
the  explanation  is  certain.  The  stars  are  partially  eclipsed  by  a  dark  companion  revolving 
round  them.  Algol  is  the  typical  star,  which  has  given  its  name  to  the  group. 

The  type  of  variation  of  a  variable  star  is  best  shown  by  the  light  curve.  The  light 
curve  of  an  invariable  star  is  a  horizontal  straight  line.  A  decrease  of  magnitude  is  shown 
by  the  line  dipping  down,  and  if  the  curve  is  carefully  drawn,  so  that  equal  distances 
horizontally  denote  equal  intervals  of  time,  while  equal  intervals  vertically  denote  equal 
changes  of  magnitude,  the  curve  is  a  complete  representation  of  the  light  variability. 

Thus,  for  Algol,  the  light  curve  is  drawn  thus  : — 


Mag. 

ao- 


30 


I9O3 
Dec  IO  II  12  13  14  IS  16  17  18 


52 


POPULAR   GUIDE   TO   THE   HEAVEXS. 


and  the  interpretation  is  as  follows.  Daring  the  greater  part  of  the  time  the  light  of 
Algol  is  uniformly  of  magnitude,  2'3 ;  but  every  2  d.  20  h.  49  m.  the  star  drops  down 
to  magnitude  3'6,  and  without  pausing  regains  its  original  light,  the  whole  change  occupying 
9h.  20m.  The  figure  further  shows  minima  are  predicted  for  1903  (December  12th,  at 
about  9  o'clock  in  the  evening,  and  for  December  15th,  at  6  o'clock). 
The  three  Algol  variables  which  are  visible  to  the  naked  eye  are  : — 

Name.  R.A.  Decl.  *W%«.  Period. 

j8  Persei  (Algol) 3h-    0™-    +40°  29'       2'3  to  3'6         2d-  20h-  49m 

X  Tauri   3     54       +    12     9         3'4  to  4'2         3     22     52 

^Librae 14     55  82         5'0  to  6.2         2       7     51 

These  may  be  readily  found  from  the  star  maps ;  and  the  times  when  they  undergo 
eclipse  may  be  found  in  the  Companion  to  the  Observatory. 

SHORT  PERIOD  VARIABLES. 

Our  second  class  comprises  stars  which  are  regularly  variable  in  periods  of  not  many 
days,  but  whose  variation  is  not  due  to  eclipse  by  a  dark  companion.  In  many  cases 
the  spectroscope  has  shown  that  these  stars  are  binary,  and  it  is  probably  so  in  all ; 
but  the  way  in  which  the  duplicity  of  the  star  explains  its  variation  is  quite  uncertain. 
The  light  variation  is  continuous,  and  by  the  shape  of  the  light  curves,  these  short 
period  variables  are  divided  into  two  classes.  In  the  first,  the  rise  to  maximum  is  steep 
and  the  fall  to  minimum  gentle  ;  in  the  second  the  curve  is  symmetrical.  The  two 
types  may  be  represented  thus  : — 


THE   STAR  MAPS. 


53 


The  following  is  a  list  of  short  period  variables  which  rise  above  naked  eye  brightness. 
They  may  be  found  upon  the  maps  whose  numbers  are  given  in  the  last  column. 

R.A.  Decl. 

h.       m. 

£  Geminorum 6    57  4-  20°  45' 

V  Puppis 7     55  -    48       5 

N  Velorum   9    28  -   56    30 

I  Carinse 9    42  -   61     57 

X  3  Sagittarii 17    40  -   27    47 

W  7l  Sagittarii 17     57  -   29     35 

YSagittarii 18    14  -   18    55 

K  Pavonis 18     45  -   67     23 

/SLyrse 18     46  +  33     13 

RLyme 18     52  +   43     48 

T>  Aquihe  19     46  +0     42 

S  Sagittse 19    51  +16     19 

TVulpecula? 20    46  +27     48 

o  Cephei    22     55  +   57     48 

In  case  the  limits  of  variation  are  not  well  determined,  the  magnitudes  are  given 
in  round  numbers. 


Range. 

Period  in  Days. 

Map. 

m.       m. 

3'7  to  4'5 

10'15 

59 

4-4  to  5'2 

2-25 

66 

3'4  to  4'4 

Not  well  known 

66 

37  to  5'2 

35'05 

66 

4     to  6 

7-01 

68 

4'8  to  5'8 

7'59 

68 

5'8  to  6'6 

577 

62 

4'0  to  5'5 

9'10 

69 

3'4  to  4'5 

12-91 

57 

4'0  to  47 

46'4 

57 

3'5  to  47 

7-18 

62 

5  '6  to  6'4 

8-38 

62 

5'5  to  6'5 

4-44 

57 

37  to  4'9 

5.37 

52 

LONG   PERIOD  VARIABLES. 

These  stars  have  periods  which  average  between  300  and  400  days,  and  only  one  regular 
variable  is  known  to  have  a  period  greater  than  600  days.  Almost  all  these  stars  show  at 
maximum  bright  lines  of  hydrogen  in  their  spectra ;  they  are  mostly  capricious  in  their 
behaviour,  rising  higher  at  some  maxima  than  at  others,  and  sinking  lower  at  some  minima. 
Nothing  is  certainly  known  as  to  the  cause  of  their  variability,  but  there  is  no  reason  to 
suppose  that  it  is  due  in  any  way  to  a  companion. 

The  more  conspicuous  of  the  long  period  variables  are  as  follows  : — 


R.A. 
h.     m. 
2     13 

6  8 

7  10 


Decl. 


Ceti  (Mira)  2  13  3°  32' 

»?     Geminorum  6  8  +  22  32 

Lo     Puppis   7  10  -  44  27 

R     Carhue  9  29  -  62  15 

R     Hydra)   13  23  -  22  40 

X     Cygni 19  46  +  32  37 

R     Cassiopeise     23  52  +  50  43 

Where  the  magnitude  given  is  followed  by  a  colon  (  : )  it  is  subject  to  irregularity. 


Range. 

Period  in 

Map. 

Days. 

•3  :  to    9  : 

331 

58 

3'2  to    4  : 

231 

59 

3'5  to    6'3 

140 

66 

5  :  to  10  : 

309 

66 

4:  to    97 

425 

61 

5  :  to  13-5 

406 

57 

5  :  to  12 

429 

52 

IRREGULARLY  VARIABLE  STARS. 

The  stars  in  the  first  three  classes  vary  in  periods  which  are,  on  the  whole,  the  less 
regularly  followed  the  longer  the  period.  We  now  come  to  a  class  of  stars  which  fluctuate 
so  irregularly  that  no  law  of  variation  has  yet  been  discovered.  In  such  cases  the  magnitudes 
given  are  generally  the  highest  and  lowest  which  have  been  observed,  and  there  is  every 
probability  that  the  range  may  be  exceeded  at  some  time  or  another. 


54 


POPULAR   GUIDE   TO   THE   HEAVENS. 


T  Ceti    

a  Cassiopeia?     

P  Persei 

t  Aiiri  "roe  

a  Orionis  

U  Hydra  

11  Carinee  (»/  Argus) 

W  Bootis 

R  Corona;  

g  Herculis 

a  Herculis    

u  Herculis    

R  Scuti  .. 


B.A. 

h.  in. 

0  16 
0  34 
2  57 

4  53 

5  49 
10  32 
10  40 

14  38 

15  44 

16  25 

17  9 

17  13 

18  41 


Decl. 

-20°  44' 

+  55  53 

+  38  22 

+  43  38 

+   7  23 

- 12  46 

-59  3 

+  27  2 

+  28  32 

+  42  8 

+  14  32 

+  33  14 

-    5  50 


Range. 

5   to  7 

2'2  to  2'8 

3'4  to  4'2 

3'0  to  4'5 

0-5  to  1-4 

4'5  to  6 

>1  to  7'4 

5'2  to  6.1 

5'8  to  13'0 

5   to  6 

3'1  to  3'9 

4'6  to  5'4 

5   to  9 


Map. 

58 
52 
53 
53 
59 
60 
67 
56 
56 
56 
62 
56 
62 


TEMPORARY  OR   "NEW"  STARS. 

So  called  "  new  "  stars  are  stars  that  have  suddenly  appeared  once,  and  then  faded  away 
They  all  seem  to  follow  essentially  the  same  course.  They  blaze  up  very  suddenly,  gradually 
fade  away,  often  with  fluctuations,  then  appear  to  turn  into  small  gaseous  nebulae,  and 
finally,  as  has  recently  been  shown,  become  merely  very  faint  stars,  very  probably  their 
original  condition.  There  is  no  theory  that  satisfactorily  accounts  for  more  than  a  part  of 
the  facts  known  about  them. 

The  following  is  a  list  of  temporary  stars  which  have  been  observed,  arranged  in  order  of 
discovery. 

Greatest  br.      R.A. 

h.   in.    1880. 

1572     Tycho's  Nova  in  Cassiopeia  >1          018 

1604     Nova  in  Serpentarius,  discovered  by  Fabricius  ...         >1         17  23 

1670    Nova  in  Vulpecula,  discovered  by  Anthelm  3'0        1943 

1848    Nova  Ophiuchi,  discovered  by  Hind   5'5        1653 

1860    Nova  Scorpii,    discovered  by  Auwers    in   the 

cluster  Messier  80 7'0        1610 

1866    Nova  Coronse,  discovered  by  Birmingham 2'0        1555 

1876    Nova  Cygni,  discovered  by  Schmidt    3'2        21  37 

1885     Nova  Andromedae,  discovered  by  Hartwig    in 

the  Andromeda  Nebula 7'0          037 

1887    Nova  Persei,  No.    1.,  discovered  on    Harvard 

photographs  by  Mrs.  Fleming,  1895  9'0        1     54 

1891     Nova  Aurigae,  discovered  by  Anderson,  January, 
1892,  and  afterwards  found  on  a  photograph 

taken  at  Harvard,  December  10th,  1891... 4 '5        5     24 

1893    Nova  Normae,  discovered  by  Mrs.  Fleming  on 

Harvard  photographs  7          1520 

1895    Nova  Carinae,  discovered  by  Mrs.  Fleming  on 

Harvard  photographs  11     3 

1895    Nova  Centauri,  discovered  by  Mrs.  Fleming  on 

Harvard  photographs  7'2         13  33 

1898  Nova  Sagittarii,  discovered  by  Mrs.  Fleming,  1899         47        18  55 

1899  Nova  Aquilse,  discovered  by  Mrs.  Fleming  on 

Harvard  photographs,  1900    7 

1901     Nova  Persei,  discovered  by  Anderson O'O  323 

1903    Nova  Geminorum,   discovered    by    Turner;   on 

Oxford  Astrographic  Chart  plates 7  636 


Decl. 

+  63°  29' 
-21  22 
+  27     1 
-12  43 

-22  41 
+  26  15 
+  42  18 

+  40  37 
+  56  9 


+  30  21 


-50     9 
-61   18 

-31     2 
-13  19 


+  43  30 


+  30     4 


Map. 

52 
62 
57 

62 

62 
56 
57 

52 
53 


53 


68 
67 

67 
62 


53 


54 


THE   STAK   MAPS.  55 

METEOR  SHOWERS. 

In  some  of  the  maps  will  be  found  an  asterisk,  closely  accompanied  by  a  date.  This 
marks  a  region  from  which  meteors  may  be  expected  about  the  date  in  question — in  the  lan- 
guage of  the  meteor  observer,  it  is  the  radiant  point  of  a  meteor  shower.  It  may  be  well, 
however,  to  caution  the  beginner  against  expecting  too  much  of  a  display  from  such  a  shower. 
For  most  o£the  points  a  dozen  meteors  in  the  night  rank  as  a  rich  display,  and  in  many  years 
the  radiants  are  almost  completely  quiescent.  Many  of  the  radiants  seem  to  be  persistent, 
furnishing  occasional  meteors  throughout  the  year  :  a  fact  for  which  no  easy  explanation  is 
forthcoming ;  for  if  the  meteors  of  one  radiant  really  belonged  to  one  system,  one  would 
oxpect  that  radiant  to  shift  during  the  year,  as  the  radiant  of  the  Perseids  is  supposed  to  do. 
For  information  as  to  the  smaller  and  sparser  showers  reference  may  be  made  to  the  memoir 
by  Mr.  Denning  (Memoirs,  Royal  Astron.  Soc.,  Vol.  LIIL).  A  list  of  radiant  points  is  given 
in  the  Companion  to  the  Observatory.  We  may  limit  ourselves  here  to  four  principal 
showers. 

The  LYRIDS.    April  19-22.     R.A.  18h.  8m.,  Declination  +  34* 

In  some  years  this  radiant  gives  a  number  of  meteors  ;  in  others  they  are  almost  entirely 
absent. 

The  PERSEIDS.    August  9-11.    R.A.  3h.  Om.,  Declination  +  57° 

These  are  the  well-known  August  meteors,  which  are,  on  the  whole,  the  most  reliable 
shower  of  the  year.  They  leave  long,  yellow  streaks.  For  a  month  before  the  date  of 
maximum,  similar  meteors  appear  from  radiants  lying  to  the  west  of  the  above  place,  and  it  is 
believed  that  they  are  a  real  case  of  a  moving  radiant. 

The  LEONIDS.  November  12-14.  R.A.  lOh.  Om.,  Declination  +  23° 
This  shower  is  visible  more  or  less  every  November,  giving  swift  meteors  with  greenish 
streaks.  For  a  number  of  centuries  they  had  given  brilliant  displays  every  33  years,  but  in 
1899  and  1900  the  expected  display  completely  failed.  It  is  fairly  certain  that  the  swarm  had 
been  perturbed  by  the  planet  Jupiter.  There  is  reason  to  believe  that  the  swarm  was 
captured  and  introduced  into  the  solar  system  by  the  planet  Uranus  in  the  year  126  A.D. 

The  ANDROMEDIDS.    November.    R.A.  Ih.  40m.,  Declination  +  43. 
This  is  the  shower  associated  with  the  lost  Biela's  Comet.     Rich  displays  may  be 
expected  with  some  certainty  every  thirteen  years.      Of  late  years  the  perturbations  of  the 
planet  upon  the  swarm  have  had  the  effect  of  throwing  back  the  date  of  maximum  and 
number  of  days,  and  it  is  impossible  at  present  to  give  exact  dates  for  the  future. 

PLATES  71  AND  72. 

As  I  have  already  pointed  out,  the  region  of  the  sky  which  corresponds  to  any  one  of 
the  general  series  of  maps,  is  indicated  by  the  dotted  lines  in  the  series  of  monthly  maps 
(Plates"  39  to  50).  This,  however,  is  chiefly  useful  at  localities  about  the  latitude  of  the 
British  Islands.  For  the  convenience  of  those  living  in  other  latitudes,  to  whom  it  is  hoped 
this  Atlas  will  recommend  itself,  as  well  as  to  enable  the  student  at  home  to  choose  the 
maps  suitable  for  his  purpose  with  greater  rapidity,  I  have  added  the  Northern  and  Southern 
Index  Maps  (Plates  71  and  72).  In  these  the  principal  constellations  are  marked,  and  the 
outlines  of  each  map  of  the  general  series,  with  the  numbers  of  the  corresponding  plates  in 
bold  figures.  Each  Index  Map  includes  from  the  Pole  to  25°  beyond  the  Equator,  so  that 
both  contain  the  series  of  Equatorial  maps.  Around  the  circumferences  is  marked  each  hour 
of  R.A.  The  Declination  is  not  indicated,  but  it  can  be  ascertained  with  sufficient  accuracy 
for  the  purpose  of  finding  the  required  map  by  remembering  that  the  Equatorial  zone  extends 
to  25°  Declination,  and  the  intermediate  zones  to  70°  Declination,  while  each  zone  overlaps 
that  above  and  below  it  by  5°. 


56  POPULAR   GUIDE   TO   THE   HEAVENS. 


PRECESSION. 

The  Precession  of  the  Equinoxes,  or  the  slow  motion  of  the  Earth's  axis,  in  Consequence 
of  which  the  intersection  of  the  Equator  with  the  Ecliptic  travels  along  the  latter,  brings 
about  a  constant  change  in  the  R.  A.  and  Declination  of  the  Stars  from  year  to  year.  It  is  thus 
clear  that  the  values  of  these  quantities  as  read  from  the  maps  will  only  be  strictly  accurate 
at  the  epoch  for  which  the  maps  are  drawn.  In  order  to  find  the  R.A.  and  Declination  for  any 
other  date,  it  is  necessary  to  apply  a  correction  for  this  precessional  effect,  and  if  it  is  desired 
to  mark  upon  the  maps  the  position  of  any  star  or  other  object  whose  co-ordinates  are  given 
for  a  date  different  from  that  of  the  Atlas,  a  similar  correction  must  be  applied. 

It  must,  however,  be  borne  in  mind  that  no  change  takes  place  from  this  cause  in  the 
relative  position  of  the  stars, — the  effect  being  merely  to  give  the  whole  system  of  Right 
Ascension  and  Declination  circles  a  shift,  and  thus  to  alter  the  positions  of  all  the  stars  with 
regard  to  them. 

For  accurate  astronomical  work,  the  correction  for  precession  must  in  general  be  computed 
to  a  small  fraction  of  a  second,  and  elaborate  tables  have  been  prepared  to  facilitate  this 
operation  ;  but  for  all  purposes  coming  within  the  scope  of  the  present  work,  the  following 
tables  will  be  found  amply  sufficient. 

That  given  on  the  next  page  contains  the  correction  to  the  R.A.  for  10  years'  precession. 
The  quantity  found  in  the  table  is  to  be  added,  with  the  sign  there  indicated  to  the  R.A.  at 
any  time,  in  order  to  obtain  the  K.A.  for  an  epoch  10  years  later,  or  it  is  to  be  subtracted  to 
find  the  R.A.  at  an  epoch  10  years  earlier.  For  intervals  other  than  10  years  a  proportional 
allowance  must  be  made.  The  top  and  bottom  lines  contain  the  Declination,  and  the  first 
and  last  columns  the  R.A. 

For  most  purposes  it  will  be  sufficient  in  finding  the  precession  to  take  the  R.A.  to  the 
nearest  whole  hour,  and  the  Declination  to  the  nearest  multiple  of  10  degrees.  If  the  star  is 
situated  in  the  Northern  Hemisphere,  we  find  its  Declination  in  the  first  or  last  line,  and  run 
the  eye  down  the  corresponding  column  till  we  reach  the  line  which  contains  the  star's  R.A.  in 
the  first  column ;  the  corresponding  figure  in  the  table  is  the  precession  in  R.A.  for  10  years. 
If  the  star  is  in  the  Southern  Hemisphere,  we  look  for  its  Declination  as  before,  but  we  find  its 
R.A.  in  the  last  column. 

The  second  table,  containing  the  correction  to  the  Declination  for  10  years,  is  still  more 
simple.  We  have  merely  to  enter  it  with  the  nearest  hour  of  R.A.  in  the  extreme  columns, 
and  we  find  in  the  central  column  the  corresponding  correction  to  the  Declination.  For  all 
R.A.'s  found  on  the  left  side  the  correction  is  positive,  and  negative  for  all  those  on  the  right 
side. 

The  signs  of  the  precessions  given  in  both  tables  show  the  correction  necessary  to  bring 
the  star's  place  up  to  a  subsequent  date ;  to  bring  it  back  to  an  earlier  date  the  signs  must 
be  altered.  The  table  of  precession  in  R.A.  extends  to  70°  north  and  south  of  the  Equator, 
so  that  it  is  applicable  to  all  the  stars  except  those  around  the  North  and  South  Poles, 
contained  in  Plates  51  and  70. 


THE   STAR  MAPS. 

TABLE  FOR  PRECESSION  IN  R.A. 


57 


R.A.forN.Decl. 

0° 

10" 

20° 

30° 

40° 

50° 

60° 

70° 

R.A.forS.Decl. 

h.        h. 

m. 

m. 

m. 

m. 

m. 

m. 

m. 

m. 

h.      h. 

18  or  18 

+  0-51 

+  0-47 

+  0-43 

+  0-38 

+  0-33 

+  0-25 

+  013 

-O'lO 

6  or    6 

19  „  17 

•51 

•47 

•43 

•39 

•33 

•26 

•14 

•08 

5    ,    7 

20  „  16 

•51 

•48 

•44 

•40 

•35 

•28 

•18 

-0-02 

4    ,    8 

21   „  15 

•51 

•48 

•45 

•42 

•38 

•32 

•24 

+  0-08 

3    ,     9 

22  „  14 

•51 

•49 

•47 

•45 

•42 

•38 

•32 

-21 

2    ,  10 

23   „  13 

•51 

•50 

•49 

•48 

•46 

•44 

•41 

•35 

1    ,  11 

0  „  12 

•51 

•51 

•51 

•51 

•51 

•51 

•51 

•51 

0    ,  12 

1    „  11 

•51 

•52 

•53 

•54 

•56 

•58 

•61 

•67 

23    ,  13 

2   .,  10 

•51 

•53 

•55 

•58 

•61 

•64 

•70 

•82 

22    ,  14 

3   „     9 

•51 

•54 

•57 

•60 

•64 

•70 

•78 

0-94 

21    ,  15 

4  „     8 

•51 

•55 

•58 

•62 

•67 

•74 

•85 

1-04 

20    ,  16 

5  „     7 

•51 

•55 

•59 

•64 

•69 

•77 

•88 

1-10 

19    ,  17 

6   „    6 

+  0-51 

+  0-55 

+  0-59 

+  0-64 

+  070 

+  078 

+  0'90 

+  1-12 

18    ,  18 

E.A.  forN.Decl. 

0° 

10° 

20° 

30° 

40l> 

50° 

60° 

70° 

R.A.forS.  Decl. 

TABLE    FOR    PRECESSION    IN   DECLINATION. 


R.A. 

Precession. 

R.A. 

h.               h. 

h.               h. 

0      or     24 

+    0'06   - 

12     or     12 

1              23 

•05 

13            11 

2              22 

•05 

14             10 

3              21 

•04 

15              9 

4              20 

•03 

16              8 

5              19 

•01 

17             7 

6              18 

•00 

18              6 

Example— The  star  Capella  is  situated  in  1880  in  R.A.  5  h.  8  m.,  Declination  +  45°'9 : 
find  what  its  R.A.  and  Declination  will  be  in  1905. 

Entering  the  first  Table  with  R.A.  5  h.,  and  Declination  50°,  we  find  10  years'  precession 
in  R.A.  is  +  077  m.  Hence  the  corresponding  correction  for  25  years  will  be  to  the  nearest 
whole  minute  +  2  m. 

Entering  the  second  Table  with  R.A.  5  h.,  we  find  10  years'  precession  in  Declination 
is  +  00-01,  hence  to  the  tenth  of  a  degree  the  correction  for  25  years  is  negligible,  so  that 
we  find  in  1905  R.A.  =  5  h.  8  m.  +  2  m.  =  5  h.  10  m.,  and  Decimation  =  +  45° '9. 

If  it  were  required  to  find  the  place  of  the  star  at  the  beginning  of  the  century  (i.e.,  80 
years  previously),  we  have  to  multiply  +  0'77  m.  and  +  0°'01  by  -  8,  and  we  find  the  cor- 
rections —  6m.  and  —  0°'l,  so  that  the  place  of  this  star  in  1800  is  R.A.  5  h.  2  m.,  Declina- 
tion +  45°'8. 

As  another  example,  let  us  find  the  R.A.  and  Declination  of  <"  Draconis  in  1940.  Its 
place  in  1880  is  17  h.  38  m. ;  +  68°'8.  We  find  from  the  Tables  —  O'lO  m.  as  correction  for  10 
years'  precession,  and  0°'00  as  the  correction  in  Declination  ;  we  thus  obtain  for  1940  R.A.  = 
17  h.  38  m.  —  0'6  m.  =  17  h.  37  m.  to  the  nearest  minute,  and  Declination  +  68°'8. 


53 


POPULAR  GUIDE   TO   THE   HEAVENS. 


Once  more,  suppose  that  in  1950  it  is  announced  that  a  comet  has  been  seen  in  R.A.  3  h. 
42'9  m.,  and  Declination  +  23°'96.  We  find  the  precession  in  R.A.  and  Declination  from  the 
tables  to  be,  for  10  years,  +  0'58  m.  and  +  0°'03.  Hence,  to  bring  the  place  back  to  1880, 
we  have  the  correction  —  4'1  m.  and  —  0°'21.  We  thus  have 


Comet's  R.A. 
h.      m. 
1950 3     42-9 

Correction  for  Precession...        —  4*1 


Comet's  Declination. 

+  23-96 
—    0-21 


18SO. 


3     33-8 


+  23-75 


That  is  to  say,  the  place  occupied  by  the  comet  is  indicated  on  these  maps  by  the  figures  just 
found  for  1880,  so  that  it  would  be  found  at  the  time  of  the  announcement  in  the  centre  of 
the  group  of  the  Pleiades. 

The  star  maps  of  this  work  were  drawn  for  the  Epoch  1880,  and  as  has  been  already 
explained,  they  no  longer  give  the  R.A.  and  Declination  of  the  stars  as  they  are  measured 
to-day.  It  will  be  convenient  if  we  add  a  table  showing  how  much  the  system  of  circles 
should  be  shifted  on  each  map  relatively  to  the  stars,  to  make  them  approximately  right  for 
the  Epoch  1900,  to  which  for  a  number  of  years  the  places  of  the  stars  will  generally  be 
referred. 

The  following  table  gives  the  number  of  the  map,  and  the  amount  the  system  of  circles 
must  be  shifted  in  millimetres. 


52.     0'4  right.     0.2  down. 

58.     0'4  right.     0.2  down. 

64.     0'4  right.     0.2  down. 

53.     0'6 

,         0.1      „ 

59.     0'4 

,         0.0 

65.     0'3 

0.1       „ 

54.     0'6 

,         0.1     up 

60.     0'4 

,         0.2     up 

66.     0'3 

0.1     up 

55.     0'4 

,         0.2      „ 

61.     0'4 

,         0.2       „ 

67.     0-4 

0.2       „ 

56.     0'3 

0.1      „ 

62.     0'4 

,         0.0 

68.     0'6 

0.1       „ 

57.     0'3 

,        0.1  down. 

63.     0'4 

,         0.2  down. 

69.     0-6 

0.1       „ 

It  will  be  seen  that  for  the  purposes  of  these  maps  the  change  is  almost  insensible. 


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(59) 


CHAPTER   VIII. 

PLATE  73. 
THE   GREAT  NEBULA  IN   ORION. 

The  Great  Nebula  which  lies  around  the  central  star  in  the  Sword  of  Orion  is  dimly  visible 
to  the  naked  eye  as  a  blurring  of  the  star  disc.  In  the  telescope  it  is  a  splendid  object,  full  of 
intricate  detail.  But  to  bring  out  its  full  beauty  we  must,  as  always  with  the  nebula?, 
photograph  it.  It  is  shown  then  as  an  object  of  extraordinary  complexity,  but  without  any 
obvious  plan.  Enclosed  in  it  are  .many  stars  which  certainly  belong  to  it,  and  are  not  merely 
seen  by  chance  in  the  same  direction  ;  for  they  share  a  typical  spectrum  whose  characteristic 
is  lines  of  helium  which  are  strong  also  in  the  nebula  itself. 

The  nebula  shown  in  the  plate  is  only  the  brighter  and  central  portion  of  a  much  larger 
structure  whose  existence  has  recently  been  made  clear,  an  immense  spiral  winding  about  the 
whole  constellation  of  Orion.  In  a  sense,  then,  the  more  familiar  nebula  is  to  be  ranked  with 
the  spiral  nebulae,  yet  it  is  clearly  distinguished  from  them  by  its  spectrum,  which  shows  that 
the  light  of  the  nebula  comes  from  luminous  gas,  hydrogen,  and  helium,  and  the  gas  hitherto 
undetected  upon  Earth  which  has  been  called  nebulum. 

Of  the  real  size  and  distance  of  the  nebula  practically  nothing  is  known,  beyond  the  fact 
that  it  is  certainly  immensely  distant  and  large.  And  in  this,  as  in  many  nebulas,  we  find 
black  holes  with  edges  surprisingly  sharp  which  are  very  hard  to  explain,  except  upon  the 
highly  speculative  assumption  that  they  represent  dark  material  structures  of  some  kind 
interposed  between  us  and  the  shining  nebula. 

In  this,  as  in  all  pictures  taken  with  reflecting  telescopes,  the  discs  of  the  brighter  stars 
are  disfigured  by  rays  which  are  of  purely  instrumental  origin. 

This  plate,  and  the  one  that  follows,  were  taken  at  the  Yerkes'  Observatory,  Chicago, 
by  Mr.  Ritchey,  with  a  reflecting  telescope  of  two  feet  aperture  made  in  the  workshops  of  the 
Observatory.  They  owe  a  part  of  their  beauty  to  the  fact  that  the  negatives  have  been 
reduced  in  the  bright  central  portions  which  are  inevitably  over-exposed  during  the  long  time  that 
is  required  to  photograph  the  faint  outlying  portions  of  the  nebula.  Only  by  such  a  process 
is  it  possible  to  show  on  one  plate  the  details  both  of  the  central  and  the  remote  parts  of  the 
nebula. 


PLATE  74. 
THE   GREAT   NEBULA  IN  ANDROMEDA. 

In  a  telescope  of  moderate  power  this  Great  Nebula  appears  almost  structureless,  a  soft 
elliptical  mass  of  light  steadily  brightening  towards  the  centre.     In  a  large  telescope,  under 


60  POPULAR  GUIDE   TO   THE   HEAVENS. 

exceptionally  fine  conditions,  its  outlying  portions  are  seen  to  be  furrowed  with  dark  passages. 
But  no  eye  has  ever  seen  directly  a  hundredth  part  of  the  structure  that  is  revealed  by  photo- 
graphy. Surrounding  the  central  nucleus,  which  remains  structureless,  are  a  number  of  fainter 
rings,  or  perhaps  more  probably  the  convolutions  of  a  spiral — it  is  almost  impossible  to  say 
which — and  the  whole  is  mottled  with  brighter  patches  and  perforated  with  dark  holes  of 
remarkable  sharpness.  The  light  of  the  nebula  is  such  as  would  be  given  by  vast  numbers 
of  stars  crowded  together — that  is  to  say,  the  spectrum  is  continuous,  the  light  is  white.  But 
in  the  present  state  of  knowledge  it  is  altogether  premature  to  argue  from  that,  that  the 
nebula  is  really  an  enormous  system  of  stars  very  far  away.  The  most  we  can  say  is  that  its 
composition  is  certainly  very  different  from  that  of  the  gaseous  nebulae. 


PLATE  75. 

THE      GREAT     STAR-CLUSTER     IN      HERCULES 
AND    A    NEBULA    IN    CYGNUS. 

The  photographs  reproduced  in  this  plate  were  taken  by  Mr.  W.  E.  Wilson,  at  his 
observatory  at  Daramona,  Westmeath,  with  a  two-foot  reflector. 

Star-clusters  may  be  roughly  divided  into  two  classes  :  loose  and  condensed.  Of  the 
former,  the  Pleiades  cluster  is  the  most  conspicuous  example.  (See  Plates  79  and  80.)  The 
stars  in  it  do  not  crowd  towards  the  centre  ;  on  the  contrary,  the  density  of  stars  in  the 
centre  of  that  cluster  is  less  than  the  density  in  the  surrounding  sky  ;  but  the  individual 
stars  are  brighter.  In  a  globular  cluster,  on  the  other  hand — such  as  the  Hercules  cluster — 
small  stars  are  so  crowded  towards  the  centre  that,  in  all  but  the  largest  instruments,  they 
become  indistinguishable  from  one  another. 


PLATE  76. 
THE  SPIRAL  NEBULA  IN   CANES  VENATICI. 

This,  the  most  famous  of  the  Spiral  Nebula,  had  its  true  character  first  recognised  by  Lord 
Rosse  with  his  great  reflector  at  Parsonstown  in  Ireland.  We  are  so  happily  situated  with 
respect  to  it  that  we  get  a  fair  side  view  of  it,  and  can  trace  in  considerable  detail  how  its 
branches  are  interlaced  and  studded  with  condensations  which  look  as  if  they  are  on  the  way 
to  become  stars.  Recent  photographic  work  has  shown  that  a  large  proportion  of  the  nebulae, 
both  known  and  hitherto  unknown,  are  spirals,  and  this  form  must  now  be  considered  almost 
the  rule  instead  of  the  exception. 

This  photograph,  and  the  two  following,  were  taken  at  the  Lick  Observatory  by  the  late 
Professor  Keeler,  with  the  three  foot  Orossley  Reflector,  which  was  mounted  at  Baling  by  the  late 
Dr.  Common,  sold  to  Mr.  Crossley,  of  Halifax,  and  afterwards  presented  by  him  to  the  Lick 
Observatory,  in  order  that  it  might  be  worked  under  skies  more  favourable  than  those  of 
England. 


NEBULA.  61 

PLATE  77. 
THE   DUMB-BELL  NEBULA. 

It  is  a  striking  illustration  of  the  power  of  photography  in  depicting  nebulae,  that  it  has 
brought  out  a  distinct  resemblance  between  the  Dumb-bell  in  Vulpecula  and  the  Ring  in 
Lyra,  which  could  hardly  have  been  suspected  from  the  visual  appearance  of  those  objects- 
If  we  imagine  the  nebulosity,  which  exists  inside  the  Ring,  to  shine  a  little  more  brightly,  so 
that  it  fills  up  the  Ring,  and  at  the  same  time  imagine  the  tendency  towards  thinning  out  at 
the  ends  of  the  longest  diameter  to  be  a  little  more  pronounced,  we  shall  see  how  easily  the 
Ring  might  be  transformed  into  the  Dumb-bell.  Both  are  gaseous,  and  both  have  a  central 
star.  It  is  difficult  to  resist  the  conclusion  that  the  two  nebuloe  are  closely  related  in  kind. 


PLATE  78. 
THE   RING   NEBULA   IN   LYRA. 

The  Ring  Nebula  in  Lyra  can  be  easily  found.  It  lies  in  the  line  from  /3  to  7  Lyrse, 
about  one  third  of  the  distance  from  /3  to  7.  It  may  be  seen  with  a  telescope  of  a  few  inches 
aperture,  but  it  is  doubtful  if  any  telescope  in  the  world,  excepting  perhaps  Lord  Rosse's 
reflector  when  in  its  finest  condition,  has  ever  shown  to  the  eye  so  much  as  is  presented  in 
the  photograph  here  reproduced,  which  was  taken  by  Prof.  Keeler  at  the  Lick  Observatory, 
with  the  Crossley  Reflector. 

The  central  star,  which  is  so  conspicuous  on  the  photograph,  is  barely  visible  in  the 
largest  telescopes.  It  is  much  brighter  photographically  than  visually,  probably  because  its 
light  is  composed  chiefly  of  those  rays  of  short  wave  length  to  which  the  plate  is  sensitive 
but  the  eye  nearly  insensitive. 

The  photograph  shows  quite  plainly  that  the  ring  is  not  uniformly  bright ;  there  are 
even  some  indications  that  it  is  composed  of  several  interlacing  or  over-lapping  rings,  and  it  is 
remarkable  how  the  ring  thins  out  at  the  ends  of  its  longest  diameter.  With  longer 
exposures  the  centre  of  the  ring  fills  up,  and  the  nebula  becomes  a  disk.  It  follows  that  the 
ring-like  appearance  is  in  a  sense  deceptive  ;  that  the  real  shape  of  the  nebula  is  something 
like  a  hollow  shell  of  gas,  of  which  the  borders  look  brighter,  perhaps,  because  one  is  then 
looking  through  a  greater  depth  of  the  shining  matter  :  but  this  is  at  best  a  conjecture. 
What  is  certainly  known  is  that  the  matter  which  shines  is  of  the  nature  of  self-luminous  gas, 
giving  a  bright  line  spectrum.  About  the  distance  and  the  real  size  of  the  object  very  little 
is  known,  but  it  is  practically  certain  that  were  our  solar  system  placed  in  the  centre  of  it,  it 
would  all  lie  within  the  space  covered  by  the  photographic  image  of  the  central  star. 

A  fine  example  of  a  nebula  with  no  central  condensation  is  that  in  Oygnus.  (No.  6992  in 
Dreyer's  New  General  Catalogue.)  Irregular  and  far-stretching  nebulae  such  as  this  are  not 
uncommon  in  the  Milky  Way.  They  are  probably  all  gaseous,  and  seem  to  belong  to  a  class 
altogether  different  from  the  spiral  nebulae. 


62  POPULAR  GUIDE  TO  THE  HEAVENS. 

PLATES  79  &  80. 
THE   PLEIADES. 

Plates  79  and  80  are  representations  of  the  Pleiades  made  with  telescope?  of  different 
types.  Plate  79  is  from  a  photograph  obtained  by  Dr.  Isaac  Eoberts  with  his  reflecting 
telescope  of  20  inches  aperture  and  98  inches  focal  length — a  ratio  of  aperture  to  focal  length 
of  about  1  :  5.  Such  an  instrument  is  the  most  efficient  kind  of  telescope  for  photographing 
faint  nebulosity,  and  with  similar  instruments  at  different  observatories  the  photographs  of 
plates  73  to  78  have  also  been  obtained.  Plate  80  is  engraved  from  a  photograph  made  at 
the  Paris  Observatory  by  the  Brothers  Henry  with  the  .refracting  telescope  of  13  inches 
aperture  and  135  inches  focal  length,  which  is  the  standard  pattern  in  use  at  eighteen  obser- 
vatories cooperating  in  making  the  photographic  chart  and  catalogue  of  the  heavens.  Such 
an  instrument  is  not  so  suitable  for  photographing  faint  nebulae  ;  but  it  makes  better  photo- 
graphs for  measurement.  And  in  presenting  a  series  of  the  best  pictures  of  some  of  the 
most  beautiful  objects  in  the  sky,  one  must  not  fail  to  call  attention  to  the  other  branch  of 
astronomical  photography,  the  making  of  plates  which  are  not  so  pictorially  effective,  but 
which  are  more  suitable  for  accurate  measurement.  Year  by  year  the  measurement  of 
photographs  replaces  the  older  method  of  measurement  at  the  telescope,  and  at  the  present 
time  (1903)  several  observatories  have  nearly  finished  their  share  of  the  great  International 
Photographic  Catalogue  of  Stars  mentioned  above,  which  will  include  the  places  of  about 
2,000,000  stars. 

The  Pleiades  cluster  is  the  finest  example  of  a  loose  cluster  of  bright  stars  intermixed 
with  nebulosity.  And  in  this  cluster  the  nebulosity  is  of  a  very  remarkable  character.  It 
takes  the  form  in  many  places  of  long  straight  wisps  connecting  directly  the  brighter  stars. 
In  examining  Plate  79  care  must  be  taken  not  to  confuse  these'  with  the  eight  symmetrical 
rays  proceeding  from  each  of  the  brighter  stars,  which  are  caused  by  an  instrumental  defect 
unavoidable  in  reflecting  telescopes.  Taking  the  key  map  to  Plate  80  as  a  guide  we  may 
trace  on  Plate  79  the  straight  extension  of  nebula  from  Electra  towards  Alcyone,  the  ray 
proceeding  from  below  Electra  straight  through  the  stars  numbered  1  and  7,  and  the 
remarkable  ray  which  runs  from  star  No.  10  in  a  straight  line  through  several  stars  above 
Alcyone  and  close  below  24.  Of  a  somewhat  different  character,  yet  still  with  a  marked 
tendency  to  arrangement  in  straight  lines,  is  the  nebula  which  involves  Maia  and  Merope. 

Even  apart  from  the  evidence  of  the  connecting  nebula,  there  is  little  doubt  that  the 
Pleiades  is  a  real  cluster  of  bright  stars,  and  not  a  chance  gathering  of  stars  seen  nearly  in  the 
same  direction  but  at  very  different  distances  from  us.  Of  relative  motion  among  the  stars  of 
the  group  there  is  little  or  none  ;  but  the  whole  group  is  drifting  together  at  the  rate  of 
about  9"  of  arc  per  century  past  the  other  stars  in  the  neighbourhood. 


PLATE  81. 

THE    MILKY    WAY,    AROUND    THE    STAR-CLUSTER 

MESSIER   n. 

This  is  a  reproduction  of  one  of  the  celebrated  photographs  of  the  "  star  clouds  "  of  the 
Milky  Way,  taken  by  Professor  Barnard  at  the  Lick  Observatory  in  1889.     His  description 


NEBULA.  63 

of  it  is  as  follows  :  "The  small  cluster,  Messier  11,  lies  on  the  upper  or  north  edge  of  the 
neck  of  the  large  cloud,  and  looks  like  a  nucleus.  The  western  side  of  the  great  cloud  has 
several  rather  sharply-marked  indentations  and  several  detached  masses  of  stars.  The  star 
j3  Aquilae,  on  the  upper  north  edge  of  the  great  head,  has  two  curious  sprays  of  stars 
extending  from  it,  giving  the  appearance  of  a  ram's  horns.  The  great  star-cloud  seems  to  be 
made  up  of  very  small  stars,  apparently  very  uniform  in  size.  Near  the  lower  right-hand 

corner  of  the  plate  is  shown  a  beautiful  bright  nebulous  star The  nebulosity  about 

this  star  is  somewhat  elliptical.  It  was  discovered  on  the  plates  of  1899,  and  is  quite 
noticeable  visually.  The  bright  star  near  the  N.E.  edge  of  the  plate  is  X  Aquilae.  The 
great  star-cloud  seems  to  stretch  out  to  and  surround  this  star." 

No  less  striking  than  the  brilliant  clouds  of  stars  are  the  dark  holes  and  lanes  which 
pierce  them.  These  sharply-defined  vacuities  are  characteristic  features  of  the  star-clouds, 
and  they  give  some  cause  for  suspicion  that  there  may  exist  in  space  regions  of  light-stopping 
material  which  cut  out  the  light  of  the  stars  beyond.  It  is  difficult  otherwise  to  account  for 
the  existence  of  so  many  well-defined  empty  spaces  in  a  field  of  stars  otherwise  so  rich. 


PLATE  82. 
NOVA  PERSEI  AND  THE  NEBULA  IN  MOTION. 

The  appearance  of  a  new  star  in  Perseus  was  first  observed  by  Dr.  Anderson  at 
Edinburgh,  on  1901,  Feb.  21,  at  G-.M.T.  14h.  40m.  The  star  was  then  of  magnitude  27. 
On  the  previous  night  a  photograph  had  been  taken  by  Mr.  Stanley  Williams  at  Brighton 
which  showed  stars  down  to  the  12th  magnitude,  but  no  trace  of  the  Nova.  It  is,  therefore, 
certain  that  in  little  more  than  24  hours  the  star  must  have  increased  in  brightness  more 
than  ten  thousand  times.  On  Plate  82  we  have  the  photograph  above  mentioned  ;  and,  for 
comparison  with  it,  a  second  photograph  taken  after  the  appearance  of  the  star.  So  promptly 
was  the  discovery  made  and  the  news  circulated  that  on  the  evening  of  February  22nd  the 
star  was  under  observation  all  over .  the  northern  hemisphere,  and  it  was  found  that  the 
brightness  was  still  increasing.  But  in  a  few  days  it  began  to  fail  rapidly — though  with 
very  strongly-marked  fluctuations — until  by  midsummer  it  was  invisible  to  the  naked  eye 
while  its  spectrum,  as  is  usual  in  such  cases,  had  become  that  of  a  gaseous  nebula. 

In  spite  of  the  enormous  amount  of  information  which  was  given  by  this  outburst — 
unsurpassed  since  the  days  of  Tycho  Brahe — it  does  not  seem  that  we  are  much  nearer  an 
understanding  of  its  cause.  But  one  thing  seems  to  be  clear ^-the  outburst  of  a  new  star  is 
not  due  to  the  collision  of  two  dark  bodies  which  are  thereby  raised  to  a  transcendant  heat. 

Almost  more  remarkable  than  the  star  itself  was  the  nebula  which  was  discovered 
around  it  in  the  autumn  of  1901.  The  first  satisfactory  photograph  of  it  was  obtained  at  the 
Yerkes  Observatory  on  September  20th.  On  November  7th  and  8th  a  photograph  obtained 
at  the  Lick  Observatory  showed  that  parts  of  the  nebula  were  in  rapid  motion  ;  and  the 
same  thing  was  found  independently  at  the  Yerkes  Observatory  on  the  9th  and  13th.  In 
the  lower  portion  of  our  plate  diagrams  drawn  to  scale  from  the  original  negatives  show 
these  changes  unmistakeably.  If  the  pointed  structures,  lettered  a  and  e,  are  compared  on 
the  two  diagrams,  and  reference  made  to  the  surrounding  stars  and  the  scale  at  the  side,  it 


64  POPULAR   GUIDE   TO   THE   HEAVENS. 

will  be  seen  at  once  that  these  points  have  moved  about  a  minute  of  arc  in  six  weeks.  Such 
a  rate  of  motion  is  unprecedented,  and  many  theories  have  been  advanced  to  account  for  it. 
The  most  generally  accepted  theory  is  that  there  was  around  the  star  a  complicated  nebula 
too  faint  to  be  photographed  until  it  was  lit  up  by  the  burst  of  light  which  proceeded  from 
the  star ;  and,  that  the  motion  which  was  observed  was  not  a  real  motion  of  the  nebula 
itself,  but  the  effect  of  successive  lighting-up  of  different  parts  of  the  nebula  as  the  light 
passed  outwards  over  it.  It  is  scarcely  possible  otherwise  to  acconnt  for  a  motion  which 
must  have  been  at  least  very  nearly  equal  to  the  velocity  of  light  itself. 


BALL'S   POPULAR   GUIDE  TO  THE    HEAVENS. 


Scale  :  Minutes  of  Arc. 
I 


Plate  73. 


fO 


20 


THE  GREAT  NEBULA  IN  ORION. 


G.  W.  RITCHEY.    2-ft.  Reflector,  YERKES  OBSERVATORY. 


BALL'S    POPULAR   GUIDE   TO   THE    HEAV 


Scale  :  Minute:  of  Arc. 

_1 L_ 


Plate  74. 


10 


20 


SO 


60 


THE  GREAT  NEBULA  IN  ANDROMEDA.    (M.  31  )  G.  W.  RITCHEY.    2-ft.  Reflector,  YERKES  OBSERVATORY 


-     L  o 


U  D 
Qi  Z 

uJ  O 


OS  "" 

UJ   < 


D 

dl 


UJ   ~ 

I  z 

H  < 


BALL'S  POPULAR  GUIDE  TO   THE    HEAVENS.  Scale   :   Minutes  of  Ar 


Plate  76. 


SPIRAL  NEBULA  IN  CANES  VENATICI.    (M.  51)  J.  E.  KEELER.    Crossley  Reflector,  LICK  OBSERVATORY. 


BALL'S  POPULAR  GUIDE  TO  THE    HFAVCNS 


Plate  77. 


Scale  :  Minutes  of  Arc. 
J | I 


THE  RING  NEBULA  IN  LYRA.    (M.  57.) 


J.  E.  KEELER.    Crossley  Reflector.  LICK  OBSERVATORY. 


BALL'S  POPULAR  GUIDE  TO  THE   HEAVENS.  Scale  :  Minutes  of  Arc. 


Plate  78. 


IO 


THE  DUMB-BELL  NEBULA  IN  VULPECULA  J.  E.  KEELER.  Crossley  Reflector.  LICK  OBSERVATORY 


BALL'S   POPULAR  GUIDE  TO  THE    HEAVENS. 


10  20 


Scale  :  Minutes  of  Arc. 

30  40 


Plate  79. 


JO  6O 


THE  NEBUL/E    IN  THE  PLEIADES. 


ISAAC    ROBERTS,    20-in.  Reflector.Crowborough.Sussex. 


V 


-•• 


'^. 


• 

^^ 


V  o' 


BALL'S   POPULAR   GUIDE   TO   THE    HEAVFNS 


Plate  81. 


a 

_L 


9 

_L 


THE  MILKY  WAY  AROUND  THE  STAR  CLUSTER  MESSIER  II    E.  E.  BARNARD,  6-in.  Portrait  Lens.  LICK  OBSERVATORY 


Plate  82 


Scale  :  Degrees. 


1901,  February  20th. 
Before  appearance  of  Nova. 


1901,  February  28th. 
The  Nova. 


NOVA  PERSEI.    Photographs  by  A.  STANLEY  WILLIAMS,  Hove,  Sussex. 


Scale  :    Squares  are  Two  Minutes  of  Arc. 


N  N 

THE  MOVING  NEBULA  SURROUNDING  NOVA  PERSEI. 
1901,  September  20th.  1901,  November   13th. 


Drawn  by  G.  W.  RITCHEY.  from  Photographs  taken  with  the  24-in.  Reflector,  YERKES  OBSERVATORY. 


A     SELECT     LIST 


OF 


STARS,     STAR     CLUSTERS,     AND 
NEBULA. 


(67) 


CHAPTER  IX. 

A    SELECT    LIST    OF    STARS,    STAR    CLUSTERS, 
AND    NEBULAE. 

In  preparing  a  list  of  objects  which  are  suitable  for  observation  with  small  instruments 
the  author  naturally  turns  in  the  first  place  to  Admiral  Smyth's  "Celestial  Cycle"  for 
suggestions ;  and  for  the  results  of  the  most  modern  work  upon  those  objects  to  Miss 
Clerke's  "  Problems  in  Astrophysics,"  Dr.  See's  "  Evolution  of  the  Stellar  Systems,"  and  Prof. 
Simon  Newcomb's  "  The  Stars  :  A  Study  of  the  Universe " ;  to  the  "  Companion  to  the 
Observatory,"  the  publications  of  the  Royal  Astronomical  Society,  and  the  scientific  journals. 
The  student  will  find  more  extensive  lists  of  interesting  objects  in  Mr.  Gore's  "  The  Stellar 
Heavens." 

An  attempt  has  been  made  in  these  notes  to  give  some  of  the  most  recent  results  for  the 
distance,  mass,  &c.  of  the  star  systems.  A  very  brief  summary  of  the  principles  from  which 
these  results  have  been  deduced  is  here  given  : — 

Distance  of  the  Stars. 

Astronomers  find  it  convenient  to  express  the  distance  of  a  star  from  the  solar  system  by 
a  quantity  which  is  the  apparent  angular  radius  of  the  Earth's  orbit  as  seen  from  the  star ; 
this  quantity  is  called  the  annual  parallax  of  the  star.  For  example,  the  annual  parallax 
of  a  Centauri,  our  nearest  stellar  neighbour,  is  0"'75  ;  that  is  to  say,  the  radius  of  the  Earth's 
orbit,  as  viewed  from  the  star,  would  subtend  an  angle  of  three-quarters  of  a  second  of  arc. 
Since  a  unit  of  length  viewed  at  a  distance  of  200,000  units  makes  an  angle  of  a  second  of 
arc  very  nearly,  we  have  the  following  convenient  rule  : — 

To  find  how  many  million  times  farther  away  than  the  Sun  any  given  star  is  : — 

Take  the  number  expressing  its  annual  parallax  in  seconds  of  arc  ;  multiply  it  by  five  ; 
and  find  the  reciprocal  of  the  product.  This  is  the  number  of  millions  required. 

For  example  :  The  parallax  of  Capella  is  0"'09.  The  reciprocal  of  5  times  0'09  is  about  2j. 
Hence  Capella  is  about  2j  million  times  as  far  away  as  the  Sun. 

Distance  of  Star  in  Light -Years. 

We  have  seen  that  astronomers  generally  express  the  distance  of  a  star  in  terms  of  its 
annual  parallax,  always  a  very  small  angle,  which  decreases  as  the  distance  of  the  star 
increases.  It  is  inconvenient  to  try  to  express  the  distances  in  the  ordinary  astronomical 
unit  of  length,  the  distance  of  the  Earth  from  the  Sun  ;  the  numbers  are  too  large.  But  the 
distance  which  light  travels  in  one  year  makes  a  unit  of  convenient  size.  Thus,  since 
according  to  the  best  determination,  the  time  taken  by  light  to  travel  the  distance  R  from 


QQ  POPULAR   GUIDE   TO   THE   HEAVENS. 

the  Earth  to  the  Sun  is  498'46  sees.,  in  one  year  (365d.  5h.  48m.  46s.)  light  travels  about 
€3,300  R. 

This  is  called  a  light-year. 

And  a  star  which  has  a  parallax  of  1"  is  distant  206,265  R. 

Hence  light  would  take  3'26  years  to  come  from  that  star  to  the  Earth. 

And  for  any  other  star  whose  parallax  expressed  in  seconds  of  arc  is  given,  the  time 

3'26 
taken  by  light  to  come  from  it  to  our  system  is        allax 

3'26 
Example  :  The  parallax  of  Capella  is  0"'09.     Hence  light  takes  -^— ^  =  about  36  years 

on  its  journey. 

Velocity  of  Star  at  Right  Angles  to  the  Line  of  Sight. 

This  can  be  found  when  the  star's  parallax  and  proper  motion  are  known.  If  the  star 
has  a  parallax  of  1"  and  a  proper  motion  of  l'',  it  moves  during  the  year  a  distance  at  right 
angles  to  the  line  of  sight  equal  to  the  radius  of  the  Earth's  orbit,' that  is,  about  92,900,000 
miles.  This  corresponds  to  a  speed  of  2'94  miles  per  second.  If  for  any  star  we  multiply 
this  number  by  the  annual  proper  motion,  and  divide  by  the  parallax,  we  get  the  velocity  of 
the  star  at  right  angles  to  the  line  of  sight. 

Example  :  a  Lyrte.     Proper  motion  0"'36  ;  parallax  O'"08.     Velocity  at  right  angles  to 

line  of  sight  =  — —  x  2 '94  =  about  13  miles  per  second. 

0'08 

Brightness  of  Stars  Compared  with  the  Sun. 

To  give  an  account  of  the  methods  of  comparing  the  relative  brightness  of  the  Sun  and 
stars  would  be  beyond  the  limits  of  the  present  work.  But  the  formula  which  represents 
the  relation  is  comparatively  simple.  If  *r  is  the  parallax  of  the  star,  m  its  magnitude, 
-and  r  the  ratio  of  its  light  to  the  light  of  the  Sun  removed  to  the  distance  of  the  star 

4  1 

logr  =  JQ  (5  log--  m.). 

Example  :  For  Procyon  we  have  mag.  0'47,  parallax  0"'33  ;  whence  log.  r  =  0'77,  r  =  5'9 
It  should  be  noticed  that  estimates  of  this  kind  are  very  much  affected  by  small 

changes  in  the  adopted  value  of  the  parallax  of  the  star,  and  are  therefore  necessarily 

rather  uncertain. 

Masses  of  the  Stars  Compared  with  the  Sun. 

Can  bs  found  only  for  binary  stars. 

When  a  binary  star  has  completed  enough  of  a  revolution  to  make  it  possible  to  compute 
all  the  circumstances  of  its  apparent  orbit,  it  is  possible  to  compute  the  shape  and  position  of 
the  real  orbit,  which  will  differ  from  the  apparent  orbit,  unless  it  happens  to  lie  square  to  the 
line  of  sight ;  we  also  know  its  size  in  seconds  of  arc.  If,  in  addition,  we  know  the  parallax, 
i.e.,  the  distance  of  the  star,  we  know  the  length  of  the  major  axis  of  the  real  orbit  in  terms 
of  the  distance  of  the  Earth  from  the  Sun. 

For  example  :  The  major  axis  of  the  real  orbit  of  the  companion  of  Sirius  about  that  star 
subtends  to  us  an  angle  8"'03  ;  the  major  axis  of  the  Earth's  orbit  round  the  Sun  subtends  at 


A    SELECT    LIST    OF    STARS,    STAR    CLUSTERS;    AND    NEBULA.  69 

the  distance  of  Sirius  the  angle  0"'37  (the  annual  parallax).    Hence  the  distance  of  its 

8 '03 
companion  from  Sirius  is    --r-    times,  =  217  times  the  distance  of  Earth  from  Sun.    The 

(j'o  I 

companion  of  Sirius  revolves  in  52'2  years.     It  follows  from  an  extension  of  Kepler's  3rd  law, 

(21'7Y! 
that  mass  of  Sirius  +  companion  =  ;        2-  mass  of  Sun  +  Earth.    The  mass  of  the  Earth  is 

\&2t  A)" 

negligible  compared  with  that  of  the  Sun,  and  we  have,  therefore,  on  reducing  the  fraction, 
Mass  of  Sirius  +  companion  =  37  times  mass  of  Sun. 

Spectroscopy. 

Without  entering  into  the  complicated  theory  and  methods  of  spectrum  analysis,  it  is 
possible  to  indicate  broadly  the  facts  upon  which  spectroscopic  determinations  of  various 
kinds  are  based. 

When  the  source  of  light  is  transparent  glowing  gas,  the  spectrum  consists  of  a  number 
of  isolated  bright  lines. 

When  the  source  of  light  is  a  glowing  solid  body,  the  spectrum  consists  of  a  continuous 
rainbow  band  of  colour,  with  no  details  at  all.  It  is  visible  from  the  red  to  the  violet ;  below 
the  red  it  can  be  detected  by  its  heating  effects  ;  beyond  the  visible  violet  it  continues  for 
some  distance  to  affect  the  photographic  plate. 

If  the  hot  solid  body  is  overlaid  by  cooler  layers  of  gas,  dark  absorption  lines  appear  in 
the  continuous  spectrum,  in  the  exact  places  where  bright  lines  would  be  seen  were  that  gas 
shining  alone. 

Upon  this  fact  rests  the  possibility  of  determining  what  substances  are  present  in  the 
vapours  surrounding  the  Sun  and  stars,  and  in  the  nebulae.  If  iron,  for  example,  is  vapourised 
in  the  electric  arc  its  spectrum  consists  of  bright  lines. 

In  the  spectrum  of  the  Sun  are  found  dark  lines  corresponding  exactly  in  position  with 
each  one  of  these  bright  lines.  The  conclusion  is  that  iron  exists  in  a  state  of  vapour  above 
the  Sun,  relatively  cooler  than  the  glowing  solid  particles  in  the  photosphere  below.  In  a 
similar  way  the  presence  of  a  great  number  of  other  elements  is  detected  in  the  Sun 
and  stars. 

Again,  if  hydrogen  is  made  to  glow  electrically  in  a  vacuum  tube  its  spectrum  consists  of 
certain  bright  lines.  In  the  spectrum  of  a  gaseous  nebula  bright  lines  are  found  in  the  same 
positions.  It  follows  that  hydrogen  is  shining  in  the  nebula. 

But  the  comparative  positions  of  these  lines  remain  fixed  only  so  long  as  the  sources  of 
light  are  at  rest  with  respect  to  one  another.  If  a  star  is  in  motion  with  respect  to  the  Earth 
all  the  lines  of  its  spectrum  are  slightly  shifted — towards  the  violet  if  the  star  is  approaching, 
towards  the  red  if  it  is  receding.  By  measuring  the  amount  of  this  shift  it  is  possible  to 
decide  what  is  the  velocity  of  approach  or  recession  of  the  star. 

Spectroscopic  Binaries. 

The  velocities  of  a  great  number  of  stars  relative  to  the  solar  system  have  been 
measured  by  this  method.  In  a  great  many  cases  the  velocity  has  been  found  to  vary 
regularly  in  a  definite  period.  The  conclusion  is  that  the  star  is  in  orbital  motion  round  the 
centre  of  gravity  of  itself  and  a  companion. 

In  the  majority  of  cases  the  companion  does  not  give  enough  light  to  affect  very  much 


70  POPULAR   GUIDE    TO   THE    HEAVENS. 

the  spectrum  of  the  principal  star.  In  some  cases,  however,  the  two  stars  are  nearly  of  the 
same  brightness.  Then  one  star  will  necessarily  be  approaching  while  the  other  is  receding, 
and  vice  versd  ;  and  the  spectrum  will  be  doubled,  each  star  will  show  its  own  set  of  dark 
lines,  which  will  alternately  close  up  on  and  open  out  from  each  other,  and  in  such  cases  the 
duplicity  of  the  star  is  obvious,  without  need  of  reference  to  the  positions  of  comparison  lines 
obtained  terrestrially. 

If  the  velocities  in  their  orbits  of  the  components  of  a  binary  are  known,  and  the  period 
of  revolution,  it  is  possible  to  calculate  the  real  size  of  the  orbits,  and  thence  the  masses  as 
above.  In  the  case  of  spectroscopic  binaries  one  cannot  usually  solve  the  problem  completely, 
but  can  determine  that  the  mass  of  the  pair  must  be  at  least  as  much  as  a  certain  quantity. 


35  Piscium.     Oh.  10m.  +  8°  13'. 

A  fine,  double  star,  6th  magnitude  white,  and  8th  magnitude  purplish.  The  components 
appear  to  be  relatively  fixed,  in  position  angle  150°,  and  distance  12". 

Globular  Cluster  47  Toucani.    Oh.  20m.  -  72°  39'. 

A  magnificent  cluster  containing  about  1,500  stars  within  a  radius  of  about  3'.  Visible 
to  the  naked  eye  as  a  hazy  star,  of  light  equivalent  to  4£  magnitude.  The  cluster  contains 
six  variable  stars. 

Nebula  M.  31  Andromedae.    Oh.  37m.  +  40°  43'. 

One  of  the  most  splendid  nebulae  in  the  sky,  but  not  very  interesting  as  a  telescopic 
object.  The  best  way  to  see  nebulae  in  the  telescope  is  to  set  the  instrument  just  ahead  of 
the  nebula  and  allow  it  to  drift  into  the  field.  If  close  attention  is  paid  it  is  possible  to  see 
in  the  Andromeda  nebula  certain  dark  and  apparently  straight  lanes ;  otherwise  the  nebula 
appears  almost  structureless,  fading  away  gradually  from  the  bright  centre.  Its  real 
complicated  structure  can  only  be  seen  well  in  the  photographs  (see  Plate  74),  where  it 
appears  as  a  fine  spiral  seen  obliquely  with  a  great  deal  of  curious  detail.  Its  spectrum  is 
continuous,  and  dark  lines  are  suspected  in  it  j  it  is  not,  therefore,  it  would  seem,  one  of  the 
transparent  gaseous  nebulae. 

i]  Cassiopeia.    Oh.  43m.  +  57°  17'. 

A  binary  star  of  rapid  motion,  and  large  parallax.  Magnitude  3£  and  7$  ;  distance  5'"68  ; 
position-angle  226°'4  (Maw,  1903'2).  Its  period  is  196  years,  and  its  parallax  is  0"20. 
From  these  data  it  may  be  concluded  that  the  mass  of  this  pair  of  suns  is  1'8  times  the  mass 
of  our  Sun,  that  their  luminosity  is  together  about  equal  to  that  of  the  Sun,  and  that  their 
mean  distance  apart  is  41  times  the  distance  of  the  Earth  from  the  Sun.  But  the  caution 
should  be  given  that  all  such  deductions  may  be  considerably  modified  by  a  small  change 
in  the  value  of  the  parallax  adopted ;  and  the  numbers  must  be  taken  as  examples  of  the 
kind  of  information  that  these  researches  will  give  us,  rather  than  as  absolutely  determined 
quantities. 

a  Ursse  Minoris  (Polaris).    lh.  23m.  +  88°  46'. 

The  Pole  Star  is  the  best  known  and  most  practically  important  star  in  the  sky.  On 
account  of  its  proximity  to  the  North  Pole  of  the  sky  it  appears  to  the  eye  to  be  almost 


A    SELECT    LIST    OF    STARS,    STAR    CLUSTERS,    AND    NEBULAE.  71 

devoid  of  the  ordinary  daily  movement  of  the  stars  about  the  Pole.  The -actual  diameter  of 
the  small'circle  which  it  describes  daily  is  2°  25'  (1903),  about  five  times  the  apparent 
diameter  of  the  Moon.  The  Pole  Star  can  easily  be  found  by  the  aid  of  the  "pointers," 
a  and  /3  Urste  Majoris.  It  is  a  standard  2nd  magnitude  star,  with  a  small  9th  magnitude 
companion  distant  19"  in  position-angle  212°. 

It  has  been  shown  quite  recently  by  Campbell  that  the  Pole  Star  is  a  spectroscopic 
binary,  with  a  period  of  very  nearly  four  days,  and  a  slow  orbital  motion  of  four  miles  per 
second.  But  irregularities  have  been  found  in  this  motion,  and  it  seems  probable  that 
Polaris  has  two  dark  companions. 

y  Arietis.     lh.  48m.  +  18°  49'. 

This  star  is  interesting  as  having  been  discovered  as  a  double  star  by  Hooke,  as  early  as 
1664,  when  he  was  observing  the  Comet  of  that  year — "a  like  instance  to  which  I  have  not 
else  met  with  in  all  the  heavens."  Magnitude  4*2,  4'4  ;  distance  8"'3  ;  position-angle  358°. 
Easily  visible  with  a  small  telescope. 

a  Piscium.     lh.  57m.  +  2°  17'. 

A  fine  double  star  ;  components  about  3  and  4  magnitude  ;  distance  3|"  ;  position-angle 
359°. 

7  Andromedae.     lh.  58m.  +  41°  51'. 

One  of  the  finest  double  stars  in  the  sky.  Magnitude  2|,  yellow,  and  5^,  blue  green. 
Distance  10"'2  ;  position-angle  62°.  The  blue  star  is  itself  a  binary,  distance  0"'45  (1903),  a 
difficult  object  at  present  in  any  telescope  less  than  12-in.  aperture.  The  period  of  this  small 
binary  is  55  years,  and  the  eccentricity  of  its  orbit  is  very  great.  In  1890  the  stars  were  only 
two  or  three-hundredths  of  a  second  apart,  and  no  telescope  could  separate  them. 

i  Trianguli.    2h.  7m.  +  29°  50'. 

An  exquisite  double  star,  of  which  the  primary  is  yellow,  5th  magnitude,  and  the 
companion  blue,  7th  magnitude.  Distance  about  3"'5,  and  position-angle  75°.  During  the 
80  years  in  which  the  star  has  been  under  observation  there  has  been  a  slight  diminution  in 
position-angle  and  increase  in  distance,  so  the  star  is  probably  a  very  slow  binary. 

The  Star  Clusters  in  Perseus.    2h.  12m.  +  56°  41'. 

A  splendid  pair  of  clusters  of  bright  stars,  visible  to  the  naked  eye  as  a  bright  patch  in 
the  Milky  way,  on  the  line  joining  a  Persei  with  S  Cas-iopei;)e,  at  about  three-fifths  of  the 
distance  from  the  former.  The  preceding  cluster  contains  two  bright  stars  of  the  7th 
magnitude  and  a  beautiful  "horse-shoe"  of  9th  and  10th  magnitude  stars.  The  cluster 
which  follows  about  3m.  on  the  same  parallel  is  not  so  fine,  but  contains  two  conspicuous 
triangles  of  stars. 

o  (Mira)  Ceti.    2h.  14m.  -  3°  26'. 

A  very  remarkable  variable  star  discovered  three  hundred  years  ago  by  the  German 
astronomer  Fabricius,  who  was  the  first  observer  of  sun  spots  with  the  telescope.  It  varies 


72  POPULAR    GUIDE    TO    THE    HEAVENS. 

in  a  period  of  330  days,  more  or  less,  from  about  the  3rd  magnitude  (on  the  average)  down  to 
9£ ;  it  is  impossible  to  define  its  behaviour  accurately  since  no  two  successive  cycles  are 
similar.  The  student  will  readily  find  its  place  with  reference  to  other  stars  from  the  charts, 
and  will  find  great  interest  in  observing  its  variations. 

0  Persei.     2h.  37m.  +  48°  49'. 

An  interesting  triple  star — A  of  the  4th,  B  and  C  of  the  10th  magnitudes.  A  and  B  are 
affected  with  the  same  proper  motion,  amounting  to  nearly  1"  annually,  and  probably  form  a 
binary  system.  Distance  17"'4,  position-angle  299°  (1900).  C  is  at  distance  80",  and  position- 
angle  225°  (1900),  and  does  not  share  the  proper  motion  of  the  other  two,  so  that  they  are 
rapidly  separating  from  it. 

7  Ceti.     2h.  38m.    +   2°  49'. 

A  beautiful  double  star,  A3'0m.  yellow,  and  B  6 '8m.  b'ue,  with  common  proper 
motion.  Probably  a  slow  binary.  Distance  3'"5  ;  position  angle  292°.  (1899). 

9  Eridani.    2h.  55m.   -   40°  41'. 

A  fine  double  star  for  southern  observers,  magnitudes  3'5  and  5'5  ;  distance  8"'2  ;  position 
angle  85°.  It  is  practically  certain  that  this  pair,  now  a  star  of  the  3rd  magnitude  to  the 
naked  eye,  is  identical  with  Ptolemy's  "  Last  of  the  river  "  and  with  the  Achernar  of  Al-Sufi, 
who  describes  it  as  of  the  first  magnitude.  This  is  one  of  the  clearest  cases  of  a  star  having 
lost  a  large  percentage  of  its  light  within  historical  times. 

]3  Persei  (Algol).    3h.  2m.  +  40°  34'. 

The  most  famous  variable  star  in  the  sky.  Every  2d.  21h.  its  light  suddenly  begins  to 
diminish  from  magnitude  2'4,  until  in  a  little  over  4  hours  it  has  sunk  to  3'6  ;  without  any 
appreciable  pause  it  then  rises  again  in  nearly  the  same  time  to  its  normal  brightness.  The 
conjecture  that  this  is  due  to  an  eclipse  by  a  dull  companion  has  been  confirmed  by  the 
spectroscope.  Algol  is  thus  a  spectroscopic  binary  whose  plane  passes  nearly  through  the 
solar  system.  This  being  determined  we  have  the  following  data :  Velocity  of  Algol  in 
orbit  26  miles  per  second  ;  radius  of  orbit  1,000,000  miles.  Diameter  of  bright  star  about 
1 ,000,000  miles ;  of  dull  companion  about  800,000.  In  default  of  a  knowledge  of  the  mass 
of  the  companion  we  cannot  determine  the  separation  in  miles  of  the  two  stars,  but  they 
must  be  relatively  very  cl%e  together,  only  a  few  million  miles  apart.  If  we  assume 
that  the  two  stars  have  the  same  density,  their  masses  are  nearly  in  the  ratio  of  2  : 1,  and 
their  distance  apart  about  3j  million  miles.  They,  like  all  eclipsing  variables,  seem  to  be  a 
great  deal  less  dense  than  the  Sua 

The  Pleiades.    Central  Star,  Alcyone.    3h.  41m.  +  23°  48'. 

The  well  known  naked  eye  cluster  of  bright  stars,  seen  to  greatest  advantage  in  a 
telescope  of  low  magnifying  power.  Most  of  the  brighter  stars  have  a  common  proper 
motion,  and  form,  without  doubt,  a  real  group.  Although  the  cluster  is  so  rich  in  bright  stars 
it  contains  actually  fewer  fainter  stars  than  equal  areas  of  the  surrounding  sky.  The  cluster 
is  full  of  nebulosity  which  has  an  apparent  connection  with  the  stars.  (See  Plate  79). 


A    SELECT    LIST    OF    STARS,    STAR    CLUSTERS,    AND    NEBUL.E.  73 

a  Tauri  (Aldebaran).    4h.  30m.   +   16°  18'. 

This  star,  Mag.  1'2,  conspicuous  for  its  ruddy  colour,  is  the  principal  object  in  the  group 
of  the  Hyades.  (Map  59).  Its  proper  motion  is  0"'19  and  parallax  0"'12,  whence  its  light 
is  about  23  times  that  of  the  Sun,  and  its  motion  across  the  line  of  sight  about  4  miles  per 
second.  An  occupation  of  Aldebaran  by  the  Moon,  which  not  infrequently  occurs, 
is  a  striking  phenomenon. 

a  Aurigae  (Capella).    5h.  9m.  +  45°  54'. 

One  of  the  brightest  stars  in  the  sky.  Newall  and  Campbell  found  independently  that 
it  is  a  spectroscopic  binary,  components  unequally  bright,  but  nearly  equal  in  mass,  moving 
in  orbits  of  radius  about  50,000,000  miles  in  a  period  of  104  days  ;  joint  mass  about  seventeen 
times  Sun.  The  parallax  of  the  stars  has  been  carefully  determined  as  0"'08  ;  and  this  implies 
that  the  stars  give  together  about  130  times  as  much  light  as  the  sun ;  they  must  there- 
fore be  much  brighter  mass  for  mass.  From  the  data  just  given  it  is  easy  to  see  that  they 
might  be  seen  telescopically  as  a  double  star  with  distance  0"'l,  and  the  Greenwich  observers 
believe  that  they  have  seen  the  star  elongated  though  not  clearly  divided. 

Proper  motion  0"'43 ;  annual  parallax  0"'08,  whence  its  velocity  at  right  angles  to  the 
line  of  sight  is  about  16  miles  per  second. 

Nebula  M.  i  Tauri.    oh.  29m.  +  21°  57'. 

The  "  Crab  "  nebula,  so  called  by  Lord  Rosse  because  of  the  claw-like  protuberances 
which  he  observed.  In  a  small  telescope  it  is  not  very  interesting  ;  but  it  is  famous  as  the 
object  which  induced  Messier  to  draw  up  his  celebrated  catalogue  of  nebulae,  by  the  numbers 
of  which  the  brighter  nebulae  are  known  to  this  day.  The  Crab  Nebula  is  No.  1. 

0  Orionis.     5h.  29m.  -  5°  28'. 

A  splendid  multiple  star  involved  in  the  brightest  part  of  the  great  nebula  in  Orion. 
Four  bright  stars,  of  magnitude  6,  7,  7i,  and  8,  are  the  well-known  "  trapezium."  There  are 
a  number  of  fainter  stars  included  in  the  group. 

(T  Orionis.     5h.  34m.  -  2°  39'. 

A  very  fine  multiple  star.  In  small  telescopes  it  presents  the  appearance  described  by 
Sir  William  Herschel  of  "  a  double-treble  star,  or  two  sets  of  treble  stars  almost  similarly 
situated."  Larger  instruments  show  a  number  of  other  stars,  and  Burnham  has  found  that 
the  brightest  star  is  itself  double,  and  a  rapid  binary. 

£  Orionis.     5h.  36m.   -  2°  0'. 

This,  the  following  of  the  three  stars  in  Orion's  belt,  is  a  fine  double,  with  a  distant 
faint  companion.  The  components  of  the  double  are  of  magnitude  2  and  6  ;  distance  2"'4  ; 
position-angle  156°,  slowly  increasing.  The  faint  companion,  magnitude  10,  is  in  position- 
angle  9°,  at  distance  57". 


74  POPULAR    GUIDE    TO    THE    HEAVENS. 

Cluster  M.  37  Aurigae.    5h.  46m.  +  32'  31'. 

A  magnificent  cluster  of  small  stars,  loose  and  little  condensed.  It  does  not  appear  that 
there  is  any  nebulosity  in  this  cluster,  though  in  small  instruments  the  crowd  of  small  stars 
presents  the  appearance  of  it,  "  Even  in  smaller  instruments  extremely  beautiful,  one  of  the 
finest  of  its  class.  Gaze  at  it  well  and  long." —  Webb. 

a  Orionis  (Betelgueuse).     5h.  50m.  4  7   23'. 

A  bright  yellowish-red  star,  whose  light  is  somewhat  variable,  about  0'9  usually.  Like 
nearly  all  stars  in  the  constellation  of  Orion  it  has  a  small  proper  motion,  0"'027  per 
annum,  and  a  small  parallax,  0"-024.  It  follows  that  this  star  gives  several  hundred  times 
the  light  of  the  Sun,  but  its  motion  across  the  line  of  sight  is  slow,  about  3  miles  per  second. 

|3  Aurigae.     oh.  52m.   +  44°  56'. 

Telescopically  a  single  star.  But  the  lines  in  its  spectrum  appear  alternately  double 
and  single  every  48  hours,  and  the  displacement  indicates  a  relative  velocity  of  150  miles  a 
second.  It  follows  that  the  star  is  double,  with  components  nearly  equally  bright,  revolving 
in  a  period  of  just  less  than  4  days.  The  orbit  is  somewhat  eccentric  ;  the  stars  are  at  least 
7,500,000  miles  apart,  and  their  combined  mass  4|  times  that  of  the  Sun. 

Star  Cluster  M.35  Geminorum.    6h.  3m.   +  24°  21. '. 

A  fine  and  bright,  but  loose  cluster  of  stars,  without  much  trace  of  the  condensation 
towards  the  centre  which  characterizes  a  globular  cluster. 

II  Monocerotis,     6h.  24m.    -   6°  57'. 

A  very  striking  triple  star,  A  of  the  5th  magnitude,  B  and  C  of  the  6th.  B  is  in  position 
angle  131°  and  distance  7" ;  C  in  120°,  distance  9|".  There  is  no  evidence  of  relative  motion 
in  this  system. 

a  Canis  Majoris  (Sirius).     6h.  41m.   -  16°  34'. 

The  brightest  star  in  the  sky,  Mag.  -  T6,  of  large  proper  motion,  and  large  parallax. 
Irregularities  in  its  proper  motion  suggested  that  the  star  must  have  a  companion,  which  was 
discovered  in  1862.  Its  magnitude  is  about  9,  but  it  is  visible  only  in  the  largest  telescopes 
being  very  hard  to  see  on  account  of  its  nearness  to  the  brilliant  primary.  In  1903'!  its 
distance  was  6"'3,  in  position  angle  128°.  The  parallax  of  Sirius  as  determined  by  Gill  and 
Elkin  is  0"'37.  The  period  of  the  companion  is  52  years,  and  mean  distance  from  Sirius  8" '03- 
From  these  data  we  may  conclude  that  the  total  mass  of  the  pair  is  3'7  times  that  of  the  Sun, 
but  their  combined  light  is  32  times  ;  that  their  distance  apart  is  22  times  that  of  the  Earth 
from  the  Sun.  From  the  irregularity  in  the  proper  motion  of  Sirius  it  may  further  be, 
shown  that  Sirius  is  only  about  twice  as  massive  as  its  companion,  though  it  is  10,000 
times  as  bright.  The  great  intensity  of  the  light  of  Sirius,  30  times  that  of  the  Sun,  with  only 
2|  times  its  mass,  and  the  dimness  of  the  companion  are  very  remarkable. 

a  Geminorum  (Castor).    7h.  28m.  -f  32°  6'. 

A  very  fine  double  star,  one  of  the  best  objects  for  small  telescopes.  Magnitude  2'0  and 
2'8  ;  distance  (1902)  5"7  ;  position  angle  223°.  Period  of  revolution  about  1,000  years.  The 


A    SELECT    LIST    OF    STARS,    STAR    CLUSTERS,    AND    NEBULA.  75 

interest  of  this  system  has  been  greatly  increased  by  the  discovery  that  the  fainter  component 
is  a  spectroscopic  binary  with  a  heavy  dark  companion.  Period  2  95  days  ;  velocity  of  bright 
star  in  orbit  22  miles  per  second  ;  radius  of  orbit  at  least  1,800,000  miles. 

a  Canis  Minoris  (Procyon).    7h.  34m.  +  5°  29'. 

One  of  our  nearer  neighbours  among  the  stars.  Annual  parallax  0'"33  ;  proper  motion 
1"'25  ;  Mag.  0'47,  whence  its  light  is  about  six  times  that  of  the  Sun,  and  its  velocity  at  right 
angles  to  the  line  of  sight  about  1 1  miles  a  second. 

A  binary  star  with  a  faint  but  relatively  very  massive  companion,  whose  presence  first 
became  known  by  the  large  irregularities  which  its  attraction  produces  in  the  motion  of  the 
principal  star.  The  disturbing  companion  was  at  last  discovered  with  the  great  Lick 
telescope  in  1895.  Its  mass  is  about  equal  to  that  of  the  Sun,  but  the  light  that  it  gives 
is  very  much  less,  perhaps  about  one-thousandth. 

4  Cancri.    Sh.  6m.  +  17°  59'. 

One  of  the  most  remarkable  multiple  stars  in  the  heavens.  It  is  composed  in  the  first 
place  of  two  stars,  A  and  B,  of  the  5  and  57  magnitude  respectively,  whose  orbit  has  been 
well  determined.  These  two  revolve  around  each  other,  in  a  period  of  60  years,  at  a  distance 
of  less  than  1",  and  are  accompanied  by  a  third  star,  C,  of  5 '5  magnitude,  which  revolves 
around  the  centre  of  gravity  of  all  in  an  opposite  direction.  From  irregularities  in  the 
motion  of  C,  which  take  place  in  a  period  of  17|  years,  it  i-i  concluded  that  it  is  but  a  satellite 
of  an  invisible  body  around  which  it  revolves  in  that  time,  describing  an  ellipse  with  a  radius 
of  about  one-fifth  of  a  second,  and  that  the  two  together  circle  around  A  and  B  in  600  or 
700  years. 

Cluster  M.44  Cancri.    Sh.  34m.  +  20°  21'. 

A  large  and  loose  cluster  of  stars  known  as  Prsesepe,  or  the  Bee-hive.  To  the  naked  eye 
it  appears  as  a  nebulous  patch  of  light  a  little  south  preceding  7  Cancri.  A  fine  object  in 
small  telescopes. 

£  Hydrae.     8h.  42m.  +  6°  50'. 

A  beautiful  triple  star.  A  and  B  are  respectively  of  the  4th  and  6th  magnitude,  and  are 
so  close  that  only  the  most  powerful  telescopes  can  separate  them.  Position-angle  23", 
distance  0"'13  (1902),  yellow.  The  companion  C  is  7th  magnitude,  blue,  in  position-angle 
234°,  distance  3"'47  (1902). 

a  Leonis  (Regulus).     lOh.  3m.  +  12°  27'. 

This  bright  star  (magnitude  1'23)  has  quite  a  large  proper  motion,  0"'27  per  annum,  but 
a  small  parallax,  0"02,  whence  it  follows  that  its  light  must  be  equal  to  that  of  1,000  of 
our  Suns. 

7  Leonis.     lOh.  14m.  +  20°  22'. 

A  very  fine  double  star,  orange  yellow.  Magnitude  2  and  4  ;  distance  3" '81  ;  position 
angle  115°7  (1903.8,  Lewis).  Binary  with  a  period  of  about  400  years. 


76  POPULAR    GUIDE    TO    THE    HEAVENS. 

Planetary  Nebula  H.  IV.  27  Hydrae.    lOh.  20m.  -  18°  8'. 

A  typical  planetary  nebula,  whose  light  is  equal  to  that  of  an  8th  magnitude  star. 
Admiral  Smyth  describes  it  as  "resembling  Jupiter  in  size,  equable  light,  and  colour," 
though  of  course  it  is  not  nearly  so  bright.  Its  spectrum  consists  of  bright  lines,  and  it  is 
therefore  gaseous. 

i]  Argus.     10h.  41m.  -  59*  10'. 

This,  one  of  the  most  remarkable  stars  in  the  sky,  set  in  the  middle  of  one  of  the  most 
remarkable  nebulae,  is  unfortunately  too  far  south  to  be  visible  in  European  latitudes. 
During  the  18th  and  early  part  of  the  19th  centuries  it  was  a  naked-eye  star  between  the 
2nd  and  4th  magnitude.  In  1837  it  rose  quickly  in  brightness  to  first  magnitude,  faded  a 
little,  and  in  1843  rose  very  nearly  to  the  brightness  of  Sirius.  In  the  following  30  years  it 
sank  steadily  to  magnitude  7£,  where  it  remains.  Its  spectrum  is  of  the  peculiar  type 
associated  with  the  temporary  stars,  and  it  seems  to  differ  from  them  principally  in  being 
semi-permanent. 

Planetary  Nebula  M.  97  Ursae  Majoris.    lib.  9m.  +  55°  34'. 

In  small  telescopes  a  faintly  luminous  disc  about  the  size  of  Jupiter.  In  very  large 
telescopes  it  appears  to  have  a  very  complicated  structure.  The  Earl  of  Rosse  found  two 
condensations  surrounded  by  spirals  in  opposite  directions,  from  which  it  obtained  the  name 
of  the  "  Owl  Nebula."  This,  like  nearly  all  planetary  nebulae,  gives  a  spectrum  of  bright 
lines,  and  is  therefore  gaseous. 

I  Ursae  Majoris.    lib.  13m.  +  32°  6'. 

A  beautiful  double  star,  rather  close  for  small  telescopes.  Magnitude  4  and  5  ;  distance 
2"'3;  position-angle  144°  (1902).  Period  about  60  years.  The  brighter  component  has  been 
shown  to  have  a  variable  velocity  in  the  line  of  sight,  which  shows  that  there  is  a  third  star 
in  the  system,  but  the  data  are  not  yet  complete. 

This  star  was  one  of  the  first  stars  recognized  as  binary,  having  components  which  move 
about  their  common  centre  of  gravity  in  accordance  with  the  law  of  universal  gravitation, 
and  it  was  actually  the  first  whose  orbit  was  computed  on  gravitational  principles. 

i  Leonis.    lib.  19m.  +  11°  5'. 

A  rather  close  double  star.  A,  4th  magnitude,  pale  yellow,  and  B,  7i  magnitude,  blue. 
Position-angle  53° ;  distance  2"'17  (1900).  There  is  considerable  relative  motion,  and  it  is 
almost  certainly  a  binary. 

24  Comae.    12h.  30m.  +  18°  56', 

A  fine,  but  wide,  double  star.  A,  5^  magnitude,  orange,  and  B,  7  magnitude,  blue. 
Position-angle  271° ;  distance  20".  The  colours  form  "  a  striking  and  beautiful  contrast." 

a  Crucis-     12h.  21m.  -  62°  33'. 

This,  the  brightest  star  in  the  Southern  Cross,  is  a  very  fine  triple  star.  Magnitudes  1'5, 
1'8,  and  6  ;  the  bright  stars  a  fairly  close  pair,  distance  5"'0,  position-angle  118°;  and  the 
fainter,  distant  90"  in  position-angle  202°.  The  parallax  of  the  bright  stars  is  0"'05. 


A    SELECT    LIST    OP    STARS,    STAR    CLUSTERS,    AND    NEBULA.  77 

y  Virginia.     12h.  37m.  -  0°  54'. 

A  most  interesting  binary  system,  consisting  of  two  stars  of  the  3rd  magnitude  ;  distance 
5"'74 ;  position-angle  328°'l  (1902).  Period  about  194  years.  Its  orbit  is  very  eccentric. 
In  1831  the  distance  was  1"'5,  and  in  1832  Sir  John  Herschel  predicted  that  within  the  next 
year  or  two  it  would  close  up  to  such  an  extent  that  "  none  but  the  very  finest  telescopes 
will  have  any  chance  of  showing  this  magnificent  phenomenon."  This  prediction  was  verified 
in  1836,  when  the  Dorpat  refractor  alone  was  able  to  elongate  the  star. 

a  Canum  Venaticorum.     12h.  51m.  +  38°  52'. 

A  very  easy  and  interesting  double,  showing  no  signs  at  present  of  a  binary  character. 
Magnitude  3  and  6  ;  distance  19" '8  ;  position-angle  227°.  This  star  was  named  Cor  Caroli 
by  Halley,  at  the  suggestion  of  the  court  physician,  who  believed  that  it  appeared  more 
brilliant  than  usual  on  the  evening  before  the  return  of  King  Charles  II.  to  London. 

£  Ursae  Majoris.    13h.  20m.  +  55°  27'. 

Probably  the  best  known  double  star  in  the  sky,  and  certainly  one  of  the  easiest  to  find 
and  most  effective  to  look  at  in  a  small  telescope.  It  is  the  middle  star  of  the  Bear's  Tail. 
Magnitude  2'1  and  4'2  ;  position-angle  147°'4  ;  distance  14"'4  ;  revolution  very  slow.  The 
larger  star  of  this  pair  is  itself  double,  though  telescopically  single.  It  was  the  first 
discovered  spectroscopic  binary.  Both  components  are  bright,  and  of  nearly  equal  magnitude. 
They  revolve  in  20  clays  14  hours,  in  an  eccentric  orbit ;  their  combined  mass  is  at  least  four 
times  that  of  the  Sun,  and  they  are  many  times  more  luminous. 

About  11'  away  is  the  5th  magnitude  star,  Alcor,  forming,  with  £  Ursse,  the  best-known 
example  of  a  naked-eye  pair. 

a  Virginia.     13h.  20m.  -  10°  38'. 

A  first  magnitude  star  whose  spectrum  is  shifted  backwards  and  forwards  every  four 
days  by  an  amount  denoting  a  revolution  at  the  rate  of  57  miles  a  second  round  the  common 
centre  of  gravity  of  itself  and  a  companion  whose  spectrum  is  just  perceptible.  Combined 
mass  at  least  2j  times  Sun. 

Globular  Cluster  «  Centauri.    13h.  21m.  -  46°  57'. 

The  finest  cluster  of  its  kind  in  the  sky,  containing  about  6,000  stars  in  a  space  of  about 
20'  diameter.  Visible  to  the  naked  eye  as  a  hazy  comet-like  object,  giving  as  much  light  as 
a  4th  magnitude  star.  The  cluster  contains  125  variable  stars,  of  which  98  have  periods  less 
than  24  hours. 

Cluster  M.  3  Canum  Venaticorum     13h.  38m.  +  28°  53'. 

A  very  fine  cluster  of  stars  of  the  llth  magnitude  and  fainter,  not  very  easily  resolvable 
with  small  telescopes.  This  cluster  is  extraordinarily  rich  in  variable  stars  ;  no  less  than  132 
out  of  900  stars  examined  are  regularly  variable,  many  of  them  in  very  short  periods. 

a  Bootis  (Arcturus).     Hh.  llm.  +  19°  42'. 

The  brightest  star  of  the  northern  sky,  with  the  very  large  proper  motion  of  2"'27  per 
year,  which  in  the  course  of  1,600  years  carries  it  across  a  space  in  the  sky  equal  the  apparent 


78  POPULAR    GUIDE    TO    THE    HEAVENS. 

diameter  of  the  Sun  or  Moon.  Yet  its  parallax  with  respect  to  the  fainter  surrounding  stars 
is  small— only  0"  026  (Chase),  whence  it  follows  that  its  velocity  at  right  angles  to  the  line  of 
light  is  about  200  miles  a  second,  and  its  light  is  many  hundred  times  that  of  the  Sun. 

a  Centauri.     14h.  33m.  -  60°  25'. 

A  splendid  binary  star.  Components  of  the  first  magnitude  ;  distance  2l'"6  ;  position- 
angle  211°  (1902).  Period  81  years.  This  is  the  nearest  star  to  the  Solar  System,  with  a 
parallax  0"75.  The  masses  of  the  stars  are  very  nearly  equal,  and  one  of  them  is  in  spectrum 
and  in  mass  an  almost  precise  counterpart  of  our  Sun.  The  semi-major  axis  of  the  orbit 
is  23'6  times  the  length  of  the  distance  from  Earth  to  Sun,  or  about  a  mean  between  the 
distances  of  Uranus  and  Neptune. 

€  Bobtis.     Hh.  41m.  +  27°  30'. 

A  most  beautiful  binary  star.  Magnitude  3,  yellow,  and  6|,  blue ;  distance  2'"65 ; 
position-angle  328°'4  (1900.5). 

£  Bootis.     14h.  47m.  +  19°  31'. 

A  very  interesting  binary  star  of  great  eccentricity  of  orbit.  A,  magnitude  45,  yellow  ; 
B,  magnitude  6'5,  purple.  According  to  See's  orbit,  published  in  1896,  the  period  is  128 
years  ;  but  inasmuch  as  the  place  predicted  from  his  orbit  for  1903.5  was  154°'7,  1"'25, 
whereas  at  that  time  it  was  measured  as  186°'9,  2" "36,  it  is  evident  that  this  orbit  requires 
modification. 

Cluster  M.  5  Librae.     15h.  14m.  +  2°  27'. 

A  globular  cluster  of  faint  stars  remarkable  for  the  number  which  are  variable,  about 
one  in  eleven,  in  periods  mostly  about  12  hours. 

a  Coronas.    16h.  llm.  +  34°  7'. 

An  interesting  binary  star,  with  a  period  probably  about  400  years.  A,  6th  magnitude, 
yellow ;  B,  7th  magnitude,  bluish.  Position-angle  210° ;  distance  4"'38  (1900).  The  stars 
were  at  their  closest  about  1830,  when  their  distance  was  only  a  little  more  than  1" ;  since 
that  time  they  have  been  gradually  opening  out,  and  will  continue  to  do  so  yet  for  about 
100  years. 

a  Scorpii.     16h.  23ra.  -  26°  13'. 

This  fine  reddish  first  magnitude  star,  the  heart  of  the  Scorpion,  has  a  green  7th 
magnitude  companion  at  a  distance  of  3",  in  position-angle  270° ;  but  it  is  not  easy  to 
see  on  account  of  the  glare  of  the  bright  star. 

£  Herculis.     16h.  38m,  +  31-  47'. 

A  rapid  binary  star,  which  has  performed  more  than  three  complete  revolutions  since  it 
was  discovered  by  Sir  William  Herschel  on  July  18th,  1782.  A  is  3rd  magnitude,  yellow  ; 
and  B  is  6th  magnitude,  bluish.  Its  period  is  about  35  years,  and  the  greatest  separation  of 


A    SELECT    LIST    OP    STARS,    STAR    CLUSTERS,    AND    NEBULAE.  79 

the  components  l£".  The  companion  passed  periastron  last  in  1899  at  a  distance  of  0"'5  ; 
since  then  it  has  opened  out  considerably,  and  a  recent  observation  gives  position-angle 
205°'6,  distance  1"'04  (1902'5).  There  is  considerable  evidence  that  the  companion  varies  in 
colour  from  red  to  blue. 

Globular  Cluster  M.  13  Herculis.     16h.  38m.  +  36°  37'. 

The  finest  globular  cluster  in  the  northern  sky,  and  effective  even  in  a  small  telescope, 
thoiigh  the  richest  parts  of  the  cluster  can  scarcely  be  resolved  in  the  largest  instruments. 
The  whole  contains  at  least  5,000  stars,  of  which  only  two  of  a  thousand  examined  proved  to 
be  variable. 

/ii  Scorpii.     16h.  45m.  -  37°  53'. 

Shown  by  the  spectroscope  to  be  a  binary  star  with  the  short  period  of  34  hours 
42  minutes.  The  two  components  have  at  maximum  a  relative  velocity  of  nearly  300  miles 
a  second ;  this  gives  their  separation  as  at  least  6,000,000  miles  ;  and  this  their  combined  mass 
as  at  least  15  times  that  of  the  Sun. 

a  Herculis.     17h.  10m.  +  14°  30'. 

One  of  the  finest  coloured  double  stars.  Magnitudes  2|,  orange,  and  6,  blue ;  distance 
4"'88  ;  position-angle  112°'l  (1901*5).  The  position-angle  is  very  slowly  diminishing. 

Nebula  M.  17  Sagittarii.     18h.  15m.  -  16°  15'. 

The  Omega  or  Horse-shoe  Nebula.  This  is  one  of  the  nebulae  that  can  be  seen  with 
comparatively  small  optical  power.  It  is  gaseous. 

Nebula  and  Cluster  M.  8  Sagittarii.    17h.  58m.  -  24°  22'. 

A  magnificent  irregular  nebula  in  a  very  rich  field  of  stars,  too  far  south  to  be  well  seen 
in  the  latitude  of  England,  where  it  rises  only  from  10°  to  15°  above  the  southern  horizon. 
In  a  fine  climate  it  is  easily  visible  to  the  naked  eye,  and  Gore  speaks  of  it  as  "  a  glorious 
object  with  a  3-in.  refractor  in  the  Punjaub." 

Planetary  Nebula  H.  IV.  37  Draconis.     17h.  59m.  +  66°  38'. 

One  of  the  most  conspicuous  planetary  nebulse  in  the  sky,  of  a  decided  pale  blue  colour, 
looking,  as  do  all  such  objects,  very  much  like  a  star  out  of  focus.  Gaseous. 

This  object  lies  close  to  the  north  pole  of  the  ecliptic. 

a  Lyrae  (Vega).    18h.  34m.  +  38°  41'. 

The  second  brightest  star  of  the  northern  sky.  A  very  white  star,  in  whose  atmosphere 
hydrogen  is  conspicuously  absorbent.  Proper  motion  0"'36  ;  annual  parallax  0"'08  ;  whence 
its  light  is  about  100  times  that  of  the  Sun,  and  its  velocity  at  right  angles  to  the  line  of 
sight  about  13  miles  per  second. 

£  Lyrae.     18h.  41m.  +  39°  30', 

A  double  star  with  components  about  3'  apart,  separated  to  a  good  eye  on  clear  moonless 
nights,  and  beautifully  seen  in  an  opera  glass.  A  3-in.  telescope  will  show  that  each  of  the 


80  POPULAR    GUIDE    TO    THE    HEAVENS 

stars  is  itself  a  double,  distance  2|"  and  3".    Between  the  two  pairs  are  three  smaller  stars 
visible  in  a  4-in.  telescope. 

Nebula  M,  57  Lyrse.     18h.  50m.  +  32°  54'. 

The  famous  Ring  Nebula,  very  easy  to  find  on  the  line  between  /3  and  y  Lyrae.  In  a 
small  telescope  it  appears  as  a  faint  ring,  "  a  nebula  with  a  hole  in  it "  ;  in  large  instruments 
it  is  seen  that  the  central  opening  is  not  entirely  clear  of  nebulosity.  Exactly  in  the  centre 
is  a  star  which  is  very  faint  in  the  largest  instruments,  but  which  photographs  comparatively 
easily.  For  the  picture  of  it  made  with  a  very  powerful  photographic  telescope  see 
Plate  77. 

ft  Cygni.     19h.  27m.  +  27°  45'. 

The  finest  coloured  double  star  in  the  sky  for  small  telescopes.  Magnitudes  3,  yellow, 
and  5£,  blue.  Distance  34"'2  ;  position-angle  55°'2. 

a  Aquilse  (Altair).     19h.  46m.  +  8°  36'. 

Magnitude  0'95  ;  parallax  0'"23  ;  whence  the  light  is  about  eight  times  that  of  the  Sun. 

a  Capricorn*.     20h.  13m.  -  12°  51'. 

A  fine  pair  of  stars  of  the  3rd  and  4th  magnitude,  about  6'  apart,  and  easily  separable  to 
the  naked  eye.  The  preceding  star  has  a  9th  magnitude  companion,  distant  about  45"  in 
position-angle  221° ;  and  the  following  a  9th  magnitude  companion,  distant  154"  in  position- 
angle  156°.  There  are  also  several  faint  closer  companions  to  these  stars,  and  the  whole  form 
a  very  fine  group. 

y  Delphini.     20h.  42m.  +  15°  46'. 

An  easy  double  star.  Magnitudes  4,  yellowish,  and  5,  bluish  ;  distance  11" '2  ;  position- 
angle  270° '6  ;  relative  motion,  if  any,  very  slow. 

6l  Cygni.     21h.  2m.  +  38°  13'. 

A  double  star.  Magnitudes  5*3  and  5'9  ;  distance  22"  ;  position-angle  125°  (1900). 
Famous  as  the  first  star  whose  distance  was  determined.  The  pair  has  a  very  large  common 
proper  motion  of  over  5"  per  year,  which  pointed  it  out  as  probably  near  to  the  Solar  System. 
The  mean  of  the  best  determinations  of  its  parallax  is  0"'39  ;  it  is  therefore,  with  one  rather 
doubtful  exception,  the  nearest  star  in  the  northern  sky.  Its  light  takes  nearly  8  £  years  to 
reach  us,  and  the  two  stars  together  give  only  about  one-tenth  the  light  of  the  Sun. 

//  Cygni.     21h.  40m.  +  28°  18'. 

A  fine  double  star,  probably  a  binary,  a  good  test  for  a  small  telescope.  Magnitudes  4  and 
5  ;  distance  2"'60 ;  position-angle  122°'l  (Lewis,  1901  '9).  There  is  a  companion,  magnitude  7£  ; 
distant  209",  in  position-angle  57°,  which  does  not  form  part  of  the  system.  Since  they  were 
observed  by  Sir  William  Herschel  in  1779,  the  pair  has  closed  up  from  a  separation  of  7"  to 
its  present  distance. 


A    SELECT    LIST    OF    STARS,    STAR    CLUSTERS,    AND    NEBULA.  81 

£  Aquarii.     22h.  24m.  -  0°  32'. 

A  well-known  and  striking  double  star,  easy  to  find  in  the  centre  of  a  triangle  of  naked- 
eye  stars  ;  probably  binary  of  long  period.  Magnitudes  4  and  4 ;  distance  3"gl ;  position- 
angle  321°  (1899'9,  Maw). 

3  Cephei.     22h.  25m.  +  57°  54'. 

A  remarkable  double  star,  of  which  the  brighter  component  is  variable  from  37m.  to 
4'9m.  in  a  period  of  5d.  8h.  48m.,  and  is  a  spectroscopic  binary  of  the  same  period.  But  the 
variation  of  light  is  not  due  to  an  eclipse,  since  at  the  time  of  minimum  the  motion  in  the  line 
of  sight  is  at  a  maximum.  The  variation  is  nevertheless  undoubtedly  due  to  the  influence  of 
the  dark  companion,  possibly  something  of  the  nature  of  a  tidal  disturbance.  This  star  is 
typical  of  quite  a  numerous  class  of  variable  stars  of  short  period,  which  are  probably  all 
spectroscopic  binaries  of  the  same  type.  All  have  the  characteristic  that  the  rise  in  light  is 
much  quicker  than  the  decline. 

(T  Cassiopeise     23h.  54m.  +  55°  12'. 

A  fine  double  star.  A,  magnitude  5,  white,  and  B,  magnitude  7i,  blue.  Position-angle 
324° ;  distance  3"'0. 


82  POPULAR   (5UIDE   TO   THE   HEAVENS. 


PLATE  83. 
STANDARD  TIME. 

As  soon  as  communication  by  railway  and  telegraph  is  established  in  a  country,  it  is 
convenient  to  adopt  throughout  the  country  a  uniform  system  of  time.  Very  usually  the 
time  adopted  has  been  at  first  the  mean  time  of  the  capital.  But  as  communication  between 
different  countries  increases,  great  inconvenience  arises  when  allowance  has  to  be  made  for  a 
difference  of  adopted  time  involving  an  odd  number  of  minutes  and  seconds.  A  large  number 
of  countries  and  states  have  therefore  adopted  a  standard  system  of  time  based  upon  that  of 
Greenwich,  and  differing  from  it  by  an  exact  number  of  hours,  with  occasionally  an  odd  half 
hour. 

Plate  83  shows  the  system  of  standard  time  adopted  throughout  the  world,  so  far  as  it 
depends  upon  Greenwich.  In  Europe  the  time  is  generally  that  of  Greenwich  or  one  hour 
fast  of  it.  Quite  recently  France  has  prepared  to  adopt  Greeewich  Time,  and  the  only 
countries  not  included  in  the  system  are  Portugal,  Russia,  Turkey,  and  Greece.  The  time 
one  hour  fast  on  Greenwich  is  known  as  Mid.  Europe  Time  ;  that  two  hours  fast  as  Eastern 
Europe  Time. 

In  the  United  States  and  Canada  there  are  five  divisions.  Inter-Colonial  or  Atlantic 
Time  is  four  hours  slow,  Eastern  or  New  York  is  five  hours,  Central  six  hours,  Mountain 
seven  hours,  and  Pacific  Time  eight  hours  slow  on  Greenwich. 

On  the  180th  meridian  the  time  is  12  hours  different  from  Greenwich,  and  provision  has 
to  be  made  for  the  change  of  date.  Since  it  would  be  very  inconvenient  to  use  a  different 
date  in  different  islands  of  the  same  group,  the  "  date  line"  does  not  follow  exactly  the  180th 
meridian,  but  is  drawn  in  a  zig-zag  course  to  avoid  land.  The  greatest  departure  from  the 
meridian  is  in  the  North  Pacific  Ocean,  where  the  line  takes  a  wide  sweep  west  to  give  the 
Aleutian  Islands  the  American  date  and  then  turns  sharply  eastward  of  the  180th  meridian 
to  avoid  the  extreme  eastern  portion  of  Siberia.  To  the  east  of  the  line  the  date  prevails 
which  has  come  round  via  America  ;  to  the  west  the  date  that  has  come  by  the  Old  World. 
Thus  in  the  extreme  east  of  Siberia  the  date  is  more  than  a  day  ahead  of  that  in  the 
Aleutian  Islands. 


(83) 


INDEX. 


PAGE. 

ABENEZRA,  Lunar  Object  No.  88,  PI.  26   30 

ABERRATION. — An  apparent  displacement  of  a 

star,  arising  from  the  progressive  movement 

of  light  combined  with  the  orbital  movement 

of  the  Earth. 
ABNEY,  Sir  W.,  Photograph  of  Solar  Corona, 

PL  16 19 

ABULPEDA,  Lunar  Object  No.  85,  PL  26,  31 30 

ACHROMATIC. — Applied    to  a    combination    of 

lenses    which    conduct    rays    of    diiferent 

colours  to  the  same  focus. 

ADAMS,  Lunar  Object  No.  17,  PI.  26 30 

^ESTUUM  SINDS,  in  Moon,  N,  PL  24,  32,  33 33 

AGATHARCHIDES,  Lunar  Object  No.  346,  PL  25, 

35    32 

AGRIPPA,  Lunar  Object  No.  244,  PL  23,  31  31 

AIRY,  Lunar  Object  No.  106,  PL  26,  31 30 

ALBATEGNIUS,  Lunar  Object  No.  109,  PL  26,  31    30 

ALCYONE,  in  Pleiades,  PL  80 62,  72 

ALDEBAEAN  or  a  TAURI,  PL  59  73 

ALEXANDER,  Lunar  Object  No.  209,  PL  23, 27,  31    31 

ALFRAGANDS,  Lunar  Object  No.  77,  PL  26 25,  30 

ALGOL,  or  /3  PERSEI,  remarkable  Variable  Star, 

PL  53  51,72 

ALGOL  VARIABLES   .51,  52 

ALHAZEN,  Lunar  Object  No.  151.  PL  23,  27 31 

ALIACENSIS,  Lunar  Object  No.  97,  PL  26,  31  ...    30 

ALMANON,  Lunar  Object  No.  86,  PL  26,  31  30 

ALPETRAGIUS.  Lunar  Object  No.  267,  PL  25,  32, 

33    31 

ALPHONSDS,  Lunar  Object  No.  265,  PL  25,  32,  33    31 
ALPINE  VALLEY,  GREAT,  Lunar  Object  No.  211, 

PL  23,  32  31 

ALPS,  Lunar   Mountains,  a,  PL  23,  24,  32,  33, 

34,  35 33 

ALTAI,  Lunar  Mountains,  A,  PL  30    33 

ALTITUDE. — The  elevation  of  a  body  above  the 

horizon,  expressed  in  angular  measure. 
ANAXAGORAS,  Lunar  Object  No.  418,  PL  24,  33    32 
ANAXIMANDER,  Lunar  Object  No.  431,  PL  24,  36    32 
ANAXIMENES,  Lunar  Object   No.  421,   PL   24, 

35,  36 .     32 


ANDERSON,  T.  D.,  Discovery  of  Nova  Persei  63 

ANDROMEDA,  PL  52,  71. 

ANDROMEDA,  Great  Nebula  in,  M.  31,  PL  18, 

52,  74 21,59,70 

ANDROMEDA,  R,  regularly  Variable  Star,  PL  52    41 

ANDROMEDA,  Nova  1885,  PL  52 54 

ANDROMEDA,  7,  a  Double  Star,  PL  52,  53  ...    71 

ANDRDMEDIDS,  Meteor  Shower 5,  55 

ANNUAL  PARALLAX,  PL  1    3 

ANNULAR  ECLIPSE  OF  SUN,  PL  14  17 

ANNULAR  NEBULA,  M.  57,  in  Lyra,  PL  57,  78    61 
ANOMALY.— The  angle  subtended  at  the  Sun  by 

a  Planet,  and  the  point  of  its  orbit  nearest 

the  Sun,  called  the  perihelion. 

ANSGARIUS,  Lunar  Object  No.  6,  PL  26 30 

ANTARES  or  a  SCORPII,  PL  68  78 

ANTLIA,  PL  66,  67. 

APENNINES,  Lunar  Mountains,  c,  PL  20,  24,  32, 

33,  34,  35    25,33 

APERTURE. — When  applied  to  a  telescope,  means 

the  diameter  of  the  object  glass. 
APHELION.— The  point  of  a  Planet's  orbit  which 

is  furthest  from  the  Sun    5 

APIANUS,  Lunar  Object  No.  99,  PL  26,  31    30 

APOGEE. — The  point  of  the  Moon's  orbit  which  is 

most  distant  from  the  Earth. 

APOLLONIUS,  Lunar  Object  No.  136,  PL  23, 27, 28    30 
APSE. — In  a  planetary  orbit  the  apses  are  the 

points  otherwise  known  as  perihelion  and 

aphelion     4 

APUS,  PL  68,  70. 

AQUARII,  £,  a  Double  Star,  PL  63 81 

AQUARIUS,  PL  63,  64,  69,  71,  72 3 

AQUILA,  PL  62,  63,  71,  72. 

,  a,  (Altair)  80 

,  »j,  regularly  Variable  Star,   PL  62        53 

,  Nova  1899    54 

ARA,  PL  68,  72. 

ARAGO,  Lunar  Object  No.  250,  PI:  23,  30 .31 

ARATUS,  Lunar  Object  No.  238,  PI .  23 31 

ARCHIMEDES,  Lunar  Object  No.  399.  PL  24,  32, 

33,34,35 25,32 


84 


INDEX. 


ARCH YTAS,  Lunar  Object  No.  206,  PI.  23    31 

ARCTURUS  or  a  BOOTIS,  PI.  55,  56,  61    77 

ARG.EUS,  Lunar  Mountain,  s,  PL  30  33 

ARGELANDER,  Durphmusterung  Atlas    51 

ARGELANDSR,  Lunar  Object  No.  107,  PI.  26 30 

ARGUS,  T)  Variable  Star  54,76 

ARIADJEUS,  Lunar  Object  No.  251,  PI.  23 31 

ARIEL,  Satellite  of  Uranus,  PI.  7     11 

ABIES.IP1.  52,  53,  58,  71,  72. 

ARIBTIS,  y,  a  Double  Star,  PI.  58  71 

ARISTARCHUS,  Lunar  Object  No.  447,  PL  24  36, 

37,38 33 

AlUSTlLLUS,  Lunar  Object  No.  259,  PL  23,  32, 

33 25,  31 

ARISTOTELES,  Lunar  Object  No.  207,  PL  23,  30, 

31    '. 31 

ARNOLD,  Lunar  Object  No.  194,  PL  23 .'  31 

ARZACHEL,  Lunar  Object  No.  266,  PL  25,  32,  33  31 

ASCENDING  NODE  of  Planetary  Orbit,  PI.  3,4...  5 

ASTEROIDS,  PL  4     6 

ASTRONOMICAL  SYMBOLS  4 

ATLAS,  Lunar  Object  No.  186,  PL  23,  28,  29,  30  31 
AURIGA,  PI.  53,  54,  71. 

AURIGA,  Cluster  in,  M.  37.  PL  53 74 

AURIGA,  a,  Capella,  Binary  Star    73 

AURIGA,  /3,   Spectroscopic  Binary 74 

AURIGJE,  Nova  1891,  PL  53  54 

AURIGA,  e,  irregularly  Variable  Star 54 

AURORA  SINUS,  on  Mars,  PL  8   12 

AUSONIA,  on  Mars,  PL  8   12 

AUSTRALE,  MARE,  in  Moon,  C,  PL  26,  27,  28  33 
AUTOLYCUS,  Lunar  Object  No.  260,  PL  23,  32, 

33    31 

Axis  MAJOR  OF  ELLIPTIC  ORBIT 4 

Axis  MINOR  OF  ELLIPTIC  ORBIT 4 

Axis,  POLAR,  PI.  1 1 

AZIMUTH. — The  angle  between  a  point  on  the 

horizon  and  the  north  or  south. 

AZOPHI,  Lunar  Object  No.  89,  PL  26 30 

AZOUT,  Lunar  Object,  No.  138,  PL  23   ..           ,.  30 


BABBAGE,  Lunar  Object  No.  434,  PI.  24 32 

BACON,  Lunar  Object  No.  126,  PL  26,  30 30 

BAILLY,  Lunar  Object  No.  316,  PL  25,  38 32 

BAILY,  Lunar  Object  No.  216,  PL  23 31 

BALL,  Lunar  Object  No.  285,  PL  25,  32 31 

BERNARD.  E.  E  .  Drawing  of  Jupiter's  Satellite 

I.,  PI.  9 13 

BARNARD,  E  E.,  Drawing  of  Saturn,  PL  10    .  .  13 


BARNARD,  E.  E.,  Photograph  of  prominences, 

PL  12 15 

BARNARD,  E.  E.,  Photograph  of  Solar  Corona 

PL  16 .'     19 

BARNARD  E.  E.,  Photograph  of  Holmes' Comet, 

PL  18 '    21 

BARNARD,  E.  E.,  Photograph  of  Milky  Way 

PI-  81 \    62 

BAROCIUS,  Lunar  Object  No.  122,  PL  26  30 

BARROW,  Lunar  Object  No.  203,  PL  23 31 

BAYER,  Lunar  Object  No.  318,  PL  25,  35,  36  ...    32 

BEAUMONT,  Lunar  Object  No.  83,  PL  26  30 

BBER,  Lunar  Object  No.  400,  PL  24  32 

BEHAIM,  Lunar  Object  No.  141    31 

BELL,  Mr.  J.  HIND.     See  Preface. 

BELLOT,  Lunar  Object  No.  39,  PL  26 30 

BBRNOUILLI,  Lunar  Object  No.  174,  PL  23   ..     .    31 

BEROSUS,  Lunar  Object  No.  171,  PL  23 31 

BERZELIUS,  Lunar  Object  No.  176,  PL  23,  28  ...    31 

BESSARION,  Lunar  Object  No.  388,  PL  24 32 

BESSEL,  Lunar  Object  No.  236,  PL  23,  30 31 

BETTINUS,  Lunar  Object  No.  309,  PL  25,  35 32 

BIANCHINJ,  Lunar  Object  No.  425,  PL  24,  35,  36    32 

BIELA,  Lunar  Object  No.  23,  PL  26    30 

BIELA'S  COMET,  Orbit  of,  and  of  Meteors  of 

November  27th,  PL  3,  4 5 

BILLY,  Lunar  Object  No.  356,  PL  25,  36  32 

BINARY  STAR.— A  Double  Star  whereof  the  two 
components  are  found  to  be  revolving 
around  each  other. 

BIRMINGHAM,  Lunar  Object  No.  415,  PL  24 32 

BIRT,  Lunar  Object  No.  276,  PL  25   31 

BLANCANUS,  Lunar  Object  No.  312,  PL  25,  34...  32 
BLANCHINUS,  Lunar  Object  No.  101,  PL  26,  31  30 

BODB,  Lunar  Object  No.  376,  PL  24 32 

BODE'S  Law  of  Planetary  Distances     6 

BOGUSLAWSKY,  Lunar  Object  No.  26,  PL  26,  28  30 
BOHNENBERGER,  Lunar  Object  No.  60,  PL  26  ...  30 
BOND,  G.  P.,  Drawing  of  Comet  of  Donati,  PL  17  21 

BOND,  G.  P.,  Lunar  Object  No.  222,  PL  23 31 

BOND,  W.  C.,  Lunar  Object  No.  204,  PL  23 31 

BONPLAND,  Lunar  Object  No.  272,  PL  25,  33  ...  31 
BOOTES,  PL  55,  56,  61,  71. 

BOOTIS,  a,  or  ARCTUHUS,  PL  55,  56,  61 -77 

BOOTIS,  e,  a  Double  Star,  PL  56 78 

BOOTIS,  5,  a  Double  Star,  PL  61 78 

BOOTIS,  W.  .Variable  Star,  PL  56  54 

BORDA,  Lunar  Object  No.  40,  PL  26 30 

BOSCOVICH,  Lunar  Object  No.  232,  PL  23, 31    25,  31 


INDEX. 


85 


BOUSSINGAULT,  Lunar  Object  No.  25.  PI.  26,  27, 

28    .' 30 

BRADLEY,  Lunar  Mountain,  10,  PL  23    33 

BKAYLEY,  Lunar  Object  No.  452,  PI.  24   33 

BRIGGS,  Lunar  Object  No.  445,  PI.  24,  37, 38  ...  33 

BRIGHTNESS  of  Stars  compared  with  Sun 68 

BROOKS'  COMET,  1893,  IV..  PI.  18 22 

BUCK,  Lunar  Object  No.  71,  PI.  26,  30 30 

BULIJALDUS,  Lunar  Object  No.  342,  PI.  25,  34  32 

BURCKHARDT,  Lunar  Object  No.  169,  PI.  23,  27  31 

BURG,  Lunar  Object  No.  215,  PI.  23 31 

BUSCHING,  Lunar  Object  No.  70,  PL  26,  30 30 

BYRGIUS,  Lunar  Object  No.  352,  PL  25 32 


CABEUS,  Lunar  Object  No.  304,  PL  25  32 

CALIPFUS,  Lunar  Object  No.  256,  PL  23,  31,  32  31 

CALLISTO,  Satellite  of  Jupiter,  PL  7  10 

CAMELOPARDALUS,  PL  51,  53,  54. 

CAMPANUS,  Lunar  Object  No.  340,  PL  25,  34  ...  32 

CANCER,  PL  54,  59,  60. 

CANCER,  Cluster  M44,  PL  60  75 

CANCRI,  £,  Multiple  Star 75 

CANES  VENATICI,  PL  55,  56. 

CANES  VENATICI,  Spiral  Nebula  in,  PL  55,  76...  60 

CANIS  MAJOR,  PL  59,  66,  71,  72. 

CANIS  MAJORIS  a,  or  SIRIUS,  PL  59   74 

CANIS  MINOR,  PL  59,  72. 

CANIS  MINORIS,    a.  (Procyon)  Binary  Star 75 

CANUM  VENATICORUM,  M.  3,  Cluster 77 

CAN  DM  VENATICORUM,  «,  (Cor  Caroli),  Double 

Star    77 

CAPE  OF  GOOD  HOPE,  R.  Observatory,  Photo- 
graph of  Comet,  1901,  I.,  PI.  19 22 

CAPELLA,  or  a  AURIGA,  PL  53    '73 

CAPELLA,  Lunar  Object  No.  57,  PL  26,  29   30 

CAPRICORNDS,  PL  63,  69,  71,  72. 

CAPRICORNI,  a,  Pair  of  Stars    80 

CAPUANUS,  Lunar  Object  No.  337,  PL  25,  34,  35  32 

CARDANUS,  Lunar  Object  No,  394,  PL  24,  38  32 
CARINA,  PL  66,  67,  70. 

CARINJE,  »;,  Variable  Star,  PL  67    54 

CARING,  I,  regularly  Variable  Star    53 

CARING,  R,  regularly  Variable  Star  53 

CARING,  Nova  1895,  PL  67    54,  76 

CARLINI,  Lunar  Object  No.  408,  PL  24,  34  32 

CARPATHIANS,  Lunar  Mountains,  d,  PL  24, 34,  35  33 

CASATUS,  Lunar  Object  No.  305,  PL  25,  35 32 

CASSINI,  Lunar  Object,  No.  258,  PI.  23,  32 31 

CASSINI,  J.  J.,  Lunar  Object  No.  422,  PL  24,  32    32 


CASSIOPEIA,  PL  51,  52,  53,  71. 

CASSIOPEIA:,  o,  Variable  Star,  PL  52 54 

CASSIOPE^;,  R,  regularly  Variable  Star,  PL    52    53 

CASSIOPEIA,  rj,  Double  Star,  PL  52   70 

CASSIOPEIA,  ff,  a  Double  Star 81 

CASSIOPEIA,  Nova  1572,  PL  52    54 

CATHARINA,  Lunar  Object  No.  81,  PL  26,  30  ...     30 
CAUCASUS,  Lunar  Mountains,  b,  PL  23,  31,  32, 

33,  34,  35    25,  33 

CAUCHY,  Lunar  Object  No.  161,  PL  23 31 

CAVALERIUS,  Lunar  Object  No.  392,  PL  24, 37,  38    32 
CAVENDISH,  Lunar  Object  No.  351,  PL  25,  36,  37    32 

CAYLBY,  Lunar  Object  No.  254,  PL  23 31 

CENSORINUS,  Lunar  Object  No.  55,  PL  26 30 

CENTADRUS,  PL  67,  68,  72. 

CENTAURI,  u>,  Globular  Cluster    77 

CENTAURI,  a,  Binary  Star 78 

CENTAURI,  Nova  1895,  PL  67  54 

CENTRAL  MERIDIAN. — The  line  joining  the  north 
point  to  the  south  point  on  one  of  the 
Monthly  Maps  in  this  Atlas. 

CEPHEI,  S,  regularly  Variable  Star,  PL  52 53,  81 

CEPHEUS,  Lunar  Object  No.  182,  PL  23    31 

CEPHEUS,  PL  51,  52,  57,  71. 

CERBERUS  (Canal),  on  Mars,  PL  8 12 

CERES,  diameter  of 8 

CETI,  T,  Variable  Star,  PL  63,  64  54 

CETI,  7,  a  Double  Star,  PL  58 72 

CETI,  v,  a  Double  Star,  PL  58 48 

CETI,  o,  (Mira),  Variable  Star,  PL  58 53,  71 

CETUS,  PL  58,  63,  64,  71,  72. 

CHAMELEON,  PL  70. 

CHACORNAC,  Lunar  Object  No.  219,  PL  23, 29,  30    31 

CHALLIS,  Lunar  Object  No.  199,  PL  23     31 

CHARONTIS  LACUS,  on  Mars,  PL  8 12 

CHARTS  FOR  SUN  SPOT  OBSERVATIONS,  PL  13  ...    16 

CHART  OF  MARS,  PL  8  11 

CHEVALLIER,  Lunar  Object  No.  185,  PL  23,  24, 

28    31 

CHRISTIAN  MAYER,  Lunar  Object  No.  205,  PI  23    31 

CICHUS,  Lunar  Object  No.  338,  PL  25,  34 32 

CIRCINUS,  PL  68. 

CIRCLE  (Great).— A  circle  which  divides  the 
sphere  into  two  equal  portions. 

CIRCLES  OF  THE  SPHERE    

CIRCUMPOLAR  STARS 3 

CLAIRAUT,  Lunar  Object  No.  123,  PL  26,  31  ...    ( 

CLAUSIUS,  Lunar  Object  No.  336,  PI.  25  32 

CLAVIUS,  Lunar  Object  No.  298,  PL  21,  25,  26, 
33,34  35  32 


86 


INDEX. 


PAGE. 

CLEOMEDES,  Lunar  Object  No.  167,  PI.  23,  27, 28  31 

CLEOSTRATUS,  Lunar  Object  No.  437,  PI.  24,  38  32 
CCELUM,  PI.  65. 

COLOMBO,  Lunar  Object  No.  34,  PI.  26,  28  30 

COLUMBA,  PI.  65,  66,  72. 
COMA  BERENICIS,  PI.  55,  61. 

COM^B  BERENICIS,  24,  a  Double  Star,  PI.  61 76 

COMET  OF  BIELA,  PI.  3 5 

COMET  OF  BROOKS,  1893,  PI.  18 22 

COMET  OF  DONATI,  Oct.  5,  1858,  PI.  17 21 

COMET  OF  PERRINE,  1902,  III.,  PI.  19  23 

COMET  OF  HOLMES,  1892,  PI.  18 21 

COMET  OF  1882,  PI.  4 _ 6 

COMET  I.,  1866,  Orbit  of,  PI.  3    5 

COMET  III.,  1862,  Orbit  of,  PI.  3    5 

COMET  I.,  1901,  PL  19  22 

COMPANION  TO  THE  OBSERVATORY 51 

COMPARATIVE  SIZES  OF  THE  PLANETS,  PI.  5 7 

CONDAMINE,  Lunar  Object  No.  423,  PI.  24,  34, 

35,36 32 

CONDORCET,  Lunar  Object  No.  140,  PI.  23,  27  31 

CONICAL  PROJECTION  FOR  STAR  MAPS  49 

CONJUNCTION. — Used  of  two  planets  when  they 

have  the  same  longitude,  viewed  from  the 

Sun. 

CONON,  Lunar  Object  No.  239,  PI.  23    31 

COOK,  Lunar  Object  No.  35,  PI.  26    30 

COPERNICUS,  Lunar  Object  No.  380,  PI.  22, 24, 

33,  34,  35   24,  32 

COR  CAHOLI,  a  Double  Star 77 

CORDILLERAS,  Lunar  Mountains 33 

CORDOBA,  Durchmusterung  Atlas  51 

COROLLA,  PI.  69. 

CORONA  BOREAHS,  PI.  56,  71. 

CORONA  OF  SUN,  PI.  16 18 

CORONA  a,  a  Double  Star,  PI.  56   78 

CORONA  II. ,  irregularly  Variable  Star,  PI.  56  54 

CORONET.,  Nova,  1866,  PI.  56  54 

COHONIUM,  Unknown  Gas  in  Sun   19 

CORVUS,  PI.  60,  61,  71,  72. 
CRATER,  PI.  60,  67,  71,  72. 
CRISIUM  MARE,  Lunar  Sea,  A,  PL  23,  27,  28, 

29,30,31  33 

CROZIEB,  Lunar  Object  No.  38,  PI.  26  30 

CRUGER,  Lunar  Object  No.  361,  PI.  25,  37  32 

CRUX,  PI.  67,  72. 

CRUCIS,  a,  Brightest  of  Southern  Cross 76 

CULMINATION, — The  passage  of  a  heavenly  body 

across  the  meridian. 

CURTIUS,  Lunar  Object  No.  132,  PI.  26,  31 30 


PAGE. 

CUVIER,  Lunar  Object  No.  125,  PI.  26  ............  30 

CYGNI,  /3,  a  Double  Star,  PL  57  .....................  80 

CYGNI,  p,  a  Double  Star,  PL  57  .....................  80 

CYGNI,  x.  regularly  Variable  Star,  PL  57  .........  53 

CYGNI,  61,  a  Double  Star,  PL  57  .....................  80 

CYGNI,  Nova  1876,  PL  57  ..............................  54 

CYGNUS,  PL  52,  57,  71. 

CYGNUS,  Nebula  in,  PL  7o   ...........................  60 

CYRILLUS,  Lunar  Object  No.  80,  PL  26,  30  ......  30 

CYSATUS,  Lunar  Object  No.  299,  PL  25,  33  ......  32 


D'ALEMBERT  MTS.,  Lunar  Object   .................. 

DANIELL,  Lunar  Object  No.  217,  PL  23  ............ 

DAMOISEAU,  Lunar  Object  No.  364,  PL  25  ...... 

DATE,  Change  of,  PL  83  ................................. 

DAVIS,  H.,  Photograph  of  Solar  Corona,  PL  16 
DAVY,  Lunar  Object  No.  269,  PL  25  ............... 

DAWES,  Lunar  Object  No.  226,  PL  23,  30  ......... 

DECLINATION.  —  The  angular  distance  of  a  celes- 

tial body  from  the  Equator  ..................... 

DEIMOS,  one  of  the  Satellites  of  Mars,  PL  7  ...... 

DELAMBRE,  Lunar  Object  No.  73,  PL  26  ......... 

DE  LA  RUE,  Lunar  Object  No.  189,  PL  23,  28,  29 
DELAUNAY,  Lunar  Object  No.  103,  PL  26  ......... 

DELISLE,  Lunar  Object  No.  406,  PL  24,  35  ...... 

DELPHIN,  PL  63,  71,  72. 

DELPHINI,  7,  a  Double  Star,  PL  63    ............... 

DELUC,  Lunar  Object  No.  297,  PL  25,  32  ......... 

DEMOCRITUS,  Lunar  Object  No.  193,  PI.  23  ...... 

DE  MORGAN,  Lunar  Object  No.  253,  PL  23  ...... 

DESCARTES,  Lunar  Object  No.  84,  PL  26  ......... 

DESCENDING  NODE  OF  ORBIT  OF  PLANET,  PL  3,  4 
DE  Vico,  Lunar  Object  No.  354,  PI.  25  ............ 

DIONE,  Satellite  of  Saturn,  PI.  7  ..................... 

DftNYSius,  Lunar  Object  No.  248,  PL  23  ......  25 

DIOPHANTUS,  Lunar  Object  No.  405,  PL  24,  35 

DIP  of  the  horizon 

DISTANCE  OF  THE  STARS   .............................. 

DIURNAL  PARALLAX  ......................  .  ............. 

DOERFEL  MOUNTAINS,  Lunar  Object  ............... 

DONATI,  Lunar  Object,  No.  105,  PL  26,  31   ...... 

DONATI'S  COMET,  Oct.  5th,  1858,  PL  17    ......... 

DOPPELMAYER,  Lunar  Object  No.  333,  PL  25,  35 

DORADO,  PI.  65,  70,  72. 

DOUGLASS,  Mr.  A.  E.,  Map  of  Mars,  PL  8   ...... 

DRACO,  PI.  51,  55,  56,  57,  71. 

DRACO,  Planetary  Nebula  in,  H  IV.  37,  PL  51, 

56,57  ................................................... 

DREBBEL,  Lunar  Object  No.  324,  PI.  25  ............ 

DUMB-BELL  NEBULA,  in  Vulpecula,  PL  77   ...... 


10 
30 
31 
30 
32 

80 
32 
31 
31 
30 

5 

32 

H 

,  31 

32 

2 
67 

2 

33 
30 


79 
32 

61 


INDEX. 


87 


EARTH,  Dimensions  of  and  position  oi  Axis,  PI.  5  7 

EARTH-LIGHT  on  Moon  17 

EARTH,  Periodic  Time  of,  PI,  3. 

ECCENTRICITY  OF  A  PLANETARY  ORBIT,  PI.  3   ...  5 

ECLIPSES,  PL  14 17 

ECLIPTIC. — The  apparent  path  of  the  Sun  among 

the  Stars,  PI.  1    1 

EDOM  PROMONTORIUM,  on  Mars,  PL  8  12 

EGEDB,  Lunar  Object  No.  210,  PI.  23 31 

EICHSTADT,  Lunar  Object  No.  353,  PI.  25,  38  ...  32 

EINMART,  Lunar  Object  No.  162,  PL  23 31 

ELECTHA,  in  Pleiades,  PI.  80 62 

ELEMENTS, — -Used  of  a  Planet's  Orbit    4 

ELGER,  MR.  T.  GWYN.     See  Preface. 

ELLIPSE. — The  form  of  a  Planetary  Orbit 4 

ELONGATION. — The  apparent  angular    distance 

of  a  body  from  its  centre  of  motion. 

ENCELADUS,  Satellite  of  Saturn,  PL  7    11 

ENCKE,  Lunar  Object  No.  386,  PI.  24,  35,  36    ...  32 

ENCKE'S  COMET,  PL  4    6 

ENDYMION,  Lunar  Object  No.  188,  PL  23,  27, 

28,  29 31 

EPIGENES,  Lunar  Object  No.  416,  PL  24,  32  32 

EQUATOR  (Celestial).— The  great  Circle  midway 

between  the  Poles,  PL  1    1 

EQUATORIAL. — A  Telescope  mounted  so  as  to 

follow  a  Star  in  its  apparent  daily  motion. 
EQUINOX. — Either  of  the  points  on  the  Equator 

at  which  the  Sun  crosses  in  its  annual  course 

among  the  Stars  1 

EQUULEDS,  PI.  63 

ERATOSTHENES,  Lunar  Object  No.  382,  PL  24,  33, 

34   32 

ERIDANUS,  PL  58,  59,  64,  65,  71,  72. 

ERIDANI,  9,  Double  Star  72 

EROS,  small  Planet,  PL  3 5 

EUCLIDES,  Lunar  Object  No.  370,  PL  25,  34,  35  32 

EUCTEMON,  Lunar  Object  No.  198,  PL  23,  29  ...  31 

EUDOXUS,  Lunar  Object  No.  208,  PL  23,  30,  31  31 

EULER,  Lunar  Object  No.  404,  PL  21,  34,  35,  36  32 

EUMENIDES  (Canal),  on  Mars,  PL  8    12 

EUROPA,  Satellite  of  Jupiter,  PI.  7     10 


FABRICIUS,  Lunar  Object  No.  45,  PI.  26,  28,  29  30 
FACUL^E. — Patches  on  the  Sun  which  are  brighter 
than  other  parts  of  the  photosphere. 

F^CUNDIT ATIS  MARE,  in  Moon ,  B,  PL  27-31 33 

FARADAY,  Lunar  Object  No.  120,  PL  26    30 

FASTIGIUM  ARYN,  on  Mars,  PL  8    12 

FAYE,  Lunar  Object  No.  104,  PL  26    30 


FENYI,  Drawings  of  prominences,  PL  12 15 

FERMAT,  Lunar  Object  No.  91,  PI.  26,30 30 

FERNELIUS,  Lunar  Object  No.  118,  PL  26,31  ...  30 

FERMICUS,  LunarObject  No.  137,  PL  23,  27 30 

FIRST  point  of  Aries  1 

FLAMSTEED,  Lunar  Object  No.  368,  PL  25, 35, 36  32 

Foci  of  an  elliptic  orbit    4 

Focus. — A  point  where  converging  rays  meet. 
FOMALHAUT  (a  Piscis  Australis),  PL  64,  72 

FOOTANA,  Lunar  Object  No.  359,  PL  25,  36,  37  32 
FONTENELLE,  Lunar  Object  No  419,  PL  24,  33, 

34,  36 32 

FORNAX,  PL  65. 

FOUCAULT,  Lunar  Object  No.  428,  P1.24  ...' 32 

FOURIER,  Lunar  Object  No.  331,  PL  25 32 

FRACASTORIUS,  Lunar  Object  No.  62,  PL  26,  29, 

30  30 

FBA  MAURO,  Lunar  Object  No.  273,  PI.  25,  33  31 

FRIGORIS,  MARE,  in  Moon,  PL  23,  31-36,  38 33 

FRANKLIN,  Lunar  Object  No.  181,  PL  23, 28......  31 

FRAUNHOFER,  Lunar  Object  No.  19,  PL  26,  27  30 
FURNERIUS,  Lunar  Object  No.  14,  PL  26,  27,  28 

29    .  .30 


GALILEO,  Lunar  Object  No.  453,  PL  24 33 

GAMBART,  Lunar  Object  No.  372,  PI  24,  33 32 

GANGES  (Canal),  on  Mars,  PL  8  12 

GANYMEDE,  Satellite  ol  Jupiter,  PL  7  ....  10 

GASSENDI,  Lunar  Object  No.  347,  PL  25,  35,  36  32 

GARTNER,  LunarObject  No.  192,  PL 23 31 

GAURICUS,  Lunar  Object  No.  282,  PL  25,  33 31 

GAUSS,  Lunar  Object  No.  172,  PL  23,  27, 28 31 

GAY  LUSSAC,  Lunar  Object  No.  383,  PL  24,  33, 

34,35     32 

GEBER,  Lunar  Object  No.  87,  PL  26  30 

GEMINI,  PL  54,  59,  71,  72. 

GEMINORUM,  a  (Castor),  a  Double  Star,  PL  54  74 

GEMINORUM,  r],  Variable  Star,  PL  54,  59 53 

GEMINORUM,  £,  regularly  Variable  Star,   PL  54, 

59    53 

GEMINORUM,  M.  35,  Star  Cluster    74 

GEMINORUM,  Nova  1903,  PL  54  54 

GEMINUS,  Lunar  Object  No.  173,  PL  23,  27 31 

GEMMA  FRISIUS,  Lunar  Object  No.  95,  PL  26 ...  30 

GERARD,  Lunar  Object  No.  441,  PL  24 33 

GIBBOUS,  Planet  when  so  called,  PL  6   8 

GIOJA,  Lunar  Object  No.  201,  PL  23 31 

GODENIUS,  Lunar  Object  No.  31,  PL  26,  28,  29  30 

GODIN,  Lunar  Object  No.  245,  PL  23,  31  31 


INDEX. 


GOLDSCHMIDT,  Lunar  Object  No.  417,  PI.  24,  32 

33   32 

GREAT  ALPINE  VALLEY,  Lunar  Object  No.  211, 

PI.  23,  32  31 

GREENWICH,  Royal  Observatory,  Photograph  of 

Sun,  PI.  11  15 

GREENWICH,  Royal  Observatory,  Photograph  of 

Perrine's  Comet,  1902,  PL  19  23 

GHIMALDI,  Lunar  Object  No.  363,  PL  25,  37,  38    32 

GROVE,  Lunar  Object  No.  212,  PL  23 31 

GRUEMBERGEU,  Lunar  Object  No.  303,  PL  25  ...     32 
GRUITHUISEN,  Lunar  Object  No.  451.  PL  24,  35    33 
GRUS,  PL  64,  69,  72. 
GUERIKE,  Lunar  Object  No.  270,  PL  25,  33......     31 

GUTTEMBERG,  Lunar  Object  No.  32,  PL  26,  28, 

29    ..  .     30 


HADLEY,  Lunar  Mountain,  t    33 

H^MUS,  Lunar  Mountain,/,  PL  23,  31,  32  33 

HAGECIUS,  Lunar  Object  No.  24,  PL  26 30 

HAGEN'S  Atlas  of  Variable  Stars 51 

HAHN,  Lunar  Object  No.  170,  PL  23 31 

HAINZEL,  Lunar  Object  No.  326,  PL  25,  34,  35  32 

HALLEY,  Lunar  Object  No.  Ill,  PL  26,  31    30 

HALLEY'S  COMET,  PL  4 6 

HANNO,  Lunar  Object  No.  143,  PL  26    31 

HANSEN,  Lunar  Object  No.  150,  PL  23 31 

HANSTEEN,  Lunar  Object  No.  357,  PL  25,  36  ...  32 

HARBINGER  MOUNTAINS,  in  Moon,  o,  PL  24 33 

HARDING,  Lunar  Object  No.  440,  PI.  24   33 

HARKNESS,  W.,  Photograph  of  Solar  Corona, 

PI.  16 19 

HARPALUS,  Lunar  Object  No.  429,  PI.  24,  35  ...  32 

HASE,  Lunar  Object  No.  10,  PI.  26    30 

HAUSEN,  Lunar  Object  No.  320,  PL  25 32 

HEAVENS,  Sphere  of  the   1 

HECATJEUS,  Lunar  Object  No.  5,  PL  26,  27 30 

HEINSIDS,  Lunar  Object  No.  292,  PL  25,  33,  34  32 

HELICON,  Lunar  Object  No.  410,  PL  24.  34 32 

HELL,  Lunar  Object  No.  279,  PL  25 SI 

HELLAS,  on  Mars,  PL  8 12 

HENRY,  Photographs  of  Pleiades,  PL  80  62 

HERCULES,  PL  56,  57,  62,  71. 

HERCULES,  Globular  Cluster  in,  M.  13,  PL  75, 76  79 

HERCULES,  Lunar  Object  No.  187,  PL  23,  29  ...  31 

HERCULIS,  a,  irregularly  Variable  Star,  PL  62...  62 

HERCULIS,  o,  a  Double  Star,  PL  62   79 

HERCULIS,  g,  Variable  Star,  PL  56 54 

HBRCULIS.  u,  Variable  Star,  PI.  56    54 

HKRCULIS,  £,  a  Double  Star,  PL  56  ..  .78 


HERCYNIAN  MOUNTAINS,  in  Moon,  p,  PL  24  ...  33 

HERIGONIUS,  Lunar  Object  No.  348,  PL  25,  35  32 

HERMANN,  Lunar  Object  No.  367,  PL  25 32 

HERODOTUS,  Lunar  Object  No.  448,  PL  24,  36, 

37,38     33 

HERSCHEL,  J.  F.  W.,  Lunar  Object  No.  430,  PL 

24,  35 32 

HERSCHEL,  Lunar  Object  No.  263,  PL  25,  32  ...  31 
HERSCHEL,  CAROLINE,  Lunar  Object  No.  407, 

PI.  24,  34 32 

HESIODUS,  Lunar  Object  No.  281,  PL  25,  33  ...  31 

HEVEL,  Lunar  Object  No.  391,  PL  24,  37,  38  ...  32 

HILDA.— One  of  the  Minor  Planets,  PI.  4  6 

HIND,  Lunar  Object  No.  112,  Pi.  26,  31 30 

HIPPALUS,  Lunar  Object  No.  345,  Pl/25,  34,  35  32 
HIPPARCHUS,  Lunar  Object  No.  110,  PL  26,  31, 

32   30 

HOLMES'  COMET,  PL  18 21 

HOMMEL,  Lunar  Object  No.  51,  PL  26 30 

HOOKE,  Lunar  Object  No.  177,  PL  23  31 

HORIZON,  PL  1    2 

HOROLOGIUM,  PL  65. 

HORREBOW,  Lunar  Object  No.  454,  PI.  24 33 

HORROCKS,  Lunar  Object  No.  113,  PL  26.,  31,  32  30 

HORTENSIUS,  Lunar  Object  No.  378,  PI.  24,  35  32 
HOUR  ANGLE.  — The  angle  between  the  meridian 

and  a  great  circle  from  the  pole  to  a  celes- 
tial body. 

HOUZEAU,  Star  Places  given  by 50 

HUGGINS,  Lunar  Object  No.  146,  PL  23    31 

HUMBOLDT  MOUNTAINS,  Lunar  Object  33 

HUMBOLDTIANUM  MARE,  in  Moon,  PL  23,  27, 28, 

29,  30 33 

HUMBOLDT,  W.,  Lunar  Object  No.  12,  PL  26,  27  30 

HUMORUM  MARE,  in  Moon,  T,  PL  25,35,36,37,38  33 

HUYGENS,  Lunar  Mountains,  v,  PL  23 33 

HYADES,  a  group  in  Taurus,  PL  71,  72. 

HYDRA,  PL  59,  60,  61,  66,  67,  68,  71,  72. 

HYDRA,  Planetary  Nebula  in,  H  IV.  27,  PL  60  76 

HYDROS  R.,  regularly  Variable  Star,  PL  61 53 

HYDR*  U.,  Variable  Star,  PL  60  54 

HYDRA;  e,  a  Double  Star,  PL  60 75 

HYDRUS,  PL  64,  65,  70,  72. 

HYGINUS,  Lunar  Object  No.  243,  PL  23   •  31 

HYPATIA,  Lunar  Object  No.  72,  PL  26,  30  30 

HYPERION,  Satellite  of  Saturn,  PL  7 11 


IAPETUS,  Satellite  of  Saturn,  PL  7 11 

IMBRIUM  MARE,  in  Moon,  L,  PL  24,  32-38  33 

INDEX  TO  THE  PLANETS  ...  .    37 


INDEX. 


89 


INDUS,  PI.  64,  69,  70,  72. 

INFERIOR  CONJUNCTION 9 

INGHIRAMI,  Lunar  Object  No.  325,  PL  25, 37,  38  32 

lo,  Satellite  of  Jupiter,  PL  7  ' 10 

IRIDUM  SINUS,  in  Moon,  R,  PL  24,  34,  35,  37,  38  33 

IRREGULARLY  VARIABLE  STARS  53 

ISIDORUS,  Lunar  Object  No.  58,  PL  26,  29  ...    .  30 


JACOBI,  Lunar  Object  No.  127,  PL  26  30 

JANSEN,  Lunar  Object  No.  158,  PL  23,  28   31 

JANSSEN,  Lunar  Object  No.  46,  PL  26,  28,  29...     30 
JULIUS  C.ESAR,  Lunar  Object  No.  231,  PL  23, 

31 25,  31 

JUNO,  diameter  of 8 

JUPITER,  Dimensions  of,  PL  5 7 

JUPITER,  Drawings  of,  PL  9 12 

JUPITER,  First  Satellite,  Drawings  of,  PL  9    ...     13 

JUPITER,  Howtofind 44 

JUPITER,  Index  to    44,  45 

JUPITER  IN  OPPOSITION 38 

JUPITER,  Orbit  of,  PI,  4 6 

JUPITER,  Rotation  Period  of 7 

JUPITER,  Satellites  of,  PI.  7  ...  ...10,  11 


KANT,  Lunar  Object  No.  78,  PL  26 30 

KASTNER,  Lunar  Object  No.  2,  PL  26,  27 30 

KEARNEY,  J.,  Photograph  of  Solar  Corona,  PL  16  19 
KEELER,  J.  E. ,  Photograph  of  Spiral  Nebula  in 

Canes  Venatici,  PL  76  .f. 60 

KEELER,  J.  E.,  Photograph  of  the  Dumb-bell 

Nebula,  PL  77 61 

KEELER,  J.  E.,  Photograph  of  Ring  Nebula  in 

Lyra,  PL  78 61 

KEPLER,  Lunar  Object  No.  387,  PL  24,  35.  36...  32 

KIES,  Lunar  Object  No.  341,  PL  25,  34 32 

KINAU,  Lunar  Object  No.  130,  PL  26 30 

KIRCH,  Lunar  Object  No.  411,  PL  24 32 

KIRCHER,  Lunar  Object  No.  308,  PL  25  32 

KLAPROTH,  Lunar  Object  No.  306,  PL  25,  35  ...  32 

KRAFKT,  Lunar  Object  No.  395,  PL  24,  38  32 

KUNOWSKY,  Lunar  Object  No.  385,  PL  24,  35...  32 


LACAILLE,  Lunar  Object  No.  102,  PL  26  30 

LACERTA,  PL  52. 

LACROIX,  Lunar  Object  No.  328,  PL  25    32 

LACUS  MORTIS,  in  Moon,  PL  23,  29,  30,  31 33 

LACUS  SOMNIORUM,  in  Moon,  PI.  23,  29,  30,  31  33 


LAGUANGE,  Lunar  Object  No.  330,  PL  25,  37,  38  32 

LA  HIRE,  j,  Lunar  Mountain    33 

LALANDE,  Lunar  Object  No.  262,  PL  25,  33 31 

LAMBERT,  Lunar  Object  No.  402,  PL  24,  33,  34 

35   32 

LANDSBERG,  Lunar  Object  No.  371,  PL  24,  25, 

34,  35 32 

LANGRENUS,  Lunar  Object  No.  1,  PI.  26,  27,  28, 

29    30 

LA  PEYROUSE,  Lunar  Object  No.  142 31 

LAPLACE  PROMONTORY,  in  Moon,  u,  PL  24,  34, 

35,36,37 '. 33 

LASSELL,  Lunar  Object  No.  268,  PL  25 31 

LATITUDE.— The  angular  distance  of  a  heavenly 

body  from  the  Ecliptic. 

LAVOISIER,  Lunar  Object  No.  442,  PL  24 33 

LEE,  Lunar  Object  No.  334,  PL  25,  35  32 

LEGENDRE,  Lunar  Object  No.  11,  PL  26,  27 30 

LE  GENTIL,  Lunar  Object  No.  144,  PL  25 31 

LEHMANN,  Lunar  Object  No.  327,  PL  25,  36    ...  32 

LEIBNITZ  MOUNTAINS,  Lunar  Object 33 

LE  MONNIER,  Lunar  Object  No.  220,  PL  23,  29, 

30,35    31 

LEO,  PL  54,  55,  60,  71,  72. 
LEO  MINOR,  PL  54,  55. 

LEONIDS,  Meteor  Shower,  PI.  4  5,  55 

LEONIS,  a,  (Regulus) 75 

LEONIS,  y,  a  Double  Star,  PL  54,  60 75 

LEONIS,  i,  a  Double  Star,  PL  60 76 

LEPUS,  PL  59.  65,  71,  72. 

LETRONNE,  Lunar  Object  No.  349,  PL  25,  35,  36  32 

LEVERRIER,  Lunar  Object  No.  409,  PL  24,  34...  32 

LEXELL,  Lunar  Object  No.  286,  PI.  25  31 

LIBRA,  PL  61,  62,  68,  71,72. 

LIBRA,  Cluster  M.  5    78 

LIBRAE,  S,  Algol  Variable,  PL  61    52 

LIBRATION — The  swinging  of  the  Moon  by  which 

we  can  sometimes  see  a  margin  beyond  the 

half  which  is  commonly  directed  towards 

us. 

LICETUS,  Lunar  Object  No.  124,  PL  26,  31  30 

LICHTENBERO,  Lunar  Object  No.  444,  PL  24    ...  33 

LIGHT  YEARS  67 

LILIUS,  Lunar  Object  No.  128,  PL  26,  31 30 

LINDENAU,  Lunar  Object  No.  68. 'PL  26   30 

LINNE,  Lunar  Object  No.  237,  PL  20,  23,  25,  31  31 

LITTROW,  Lunar  Object  No.  224,  PL  23    31 

LOHRMANN,  Lunar  Object  No.  366,  PL  25,  37,  38  32 

LOHSE,  Dr.  0. ,  on  Jupiter.     See  Preface. 

LONG  PERIOD  VARIABLES  ...                53 


90 


INDEX. 


LONGITUDE.— If  a  great  circle  perpendicular  to 
the  Ecliptic  be  drawn  through  any  celestial 
body,  its  longitude  is  the  angle  from  the 
vernal  equinox  measured  towards  the  east 
to  the  foot  of  the  perpendicular. 
LONGITUDE  OF  PERIHELION,  PI.  3. 
LONGOMONTANUS,  Lunar  Object  No.  294,  PI.  25, 

33,34 / 32 

LOWELL  OBSERVATORY,  Map  of  Mars,  PI.  8 12 

LUBBOCK,  Lunar  Object  No.  30,  PI.  26,  28  30 

LUBINIEZKY,  Lunar  Object  No.  343,  PL  25,  34...     32 

LUNA;  LACUS,  on  Mars,  PL  8 12 

LUNAR  OBJECTS 28 

LUNATION. — The  period  from  one  new  Moon  to 

to  the  next.     29'5305879  days. 
LUPUS,  PL  68,  72. 
LYNX,  PI.  54. 
LYRA,  PI.  57,  71. 

LYRA,  Annular  Nebula,  PL  57,  78  61,  80 

LYRA;  a,  or  VEGA,  PL  57  79 

LYRA;  t ,  a  Double-double  Star,  PI .  57    79 

LYR.S;  j3,  regularly  Variable  Star,  PI.  57    53 

LYRA;,  R.,  Variable  Star,  PL  57 53 

LYRID  METEOR  SHOWER   .,  55 


MACLAUIUN,  Lunar  Object  No.  4,  PI.  26  30 

MACLEAR,  Lunar  Object  No.  229,  PI.  23,  30 31 

MACROBIUS,  Lunar  Object  No.  166,  PI.  23,  28  ...  31 

MADLER,  Lunar  Object  No.  59,  PL  26,  30 30 

MAGELHAENS,  Lunar  Object  No.  33,  PL  26  30 

MAGINUS,  Lunar  Object  No.  296,  PL  25,  32,  33  32 

MAI  A,  in  Pleiades,  PL  80  62 

MAIN,  Lunar  Object  No.  200,  PL  23  31 

MAIRAN,  Lunar  Object  No.  427,  PL  24,  35,  36...  32 

MALUS,  PI.  60,  66. 

MANILIUS,  Lunar  Object  No.  240,  PL  23,  31    ...  31 

MANNERS,  Lunar  Object  No.  249,  PL  23  31 

MANZINUS,  Lunar  Object  No.  54,  PL  26,  29 30 

MARALDI,  Lunar  Object  No.  160,  PL  23,  29    ...  31 

MARCO  POLO,  Lunar  Object  No.  398,  PL  24    ...  32 

MARE  AUSTRALE,  C,  in  Moon,  PL  26,  27,  28   ...  33 

MARE  CHRONIUM,  on  Mars,  PL  8   12 

MARE  CIMMERIUM,  on  Mars,  PL  8 12 

MARE  CRISIUM,  A,  in  Moon,  PL  23,  27,  28,  29, 

30,31     '. 33 

MARE  ERYTHRA;UM,  on  Mars,  PL  8    12 

MARE  FA:CUNDITATIS,    in   Moon,    B,    PL  26, 

27-31 33 

MARE  FRIGORIS,  K,  in  Moon,  PI.  23,  31,  36,  38...  33 


MARE  HUMBOLDTIANUM,  in  Moon,  D,  PL  23,  27, 

28,29,30 33 

MARE  HUMORUM,  in  Moon,  T,  PL  25,  35,  36,  37 

38    \  33 

MARE  TCARIUM,  on  Mars,  PL  8 12 

MARK  IMBRIUM,  in  Moon,  L,  PL  24,  32-38  33 

MARE  NECTARIS,  in  Moon,  F.,  PL  26,  29.  30,  31  33 
MARE  NUBIUM,  in  Moon,  Q,  PI.  25,  33,  34,  35, 

36,  37 '  33 

MARE  SERENITATIS,  in  Moon,  J,  PL  20,  23,  30, 

31,32  25  33 

.MARE  SIRENUM,  on  Mars,  PL  8  12 

MARE  SMYTHII,  in  Moon,  Y,  PL  23,  27 33 

MARE  TRANQUILLITATIS,  in  Moon,  E,  PL  23, 29, 

30,31 33 

MARE  TYRRHENIUM,  on  Mars,  PL  8   12 

MARE  VAPORUM,  in  Moon,  M,  PL  23,  31    33 

MARGAHITIFER  SINUS,  on  Mars,  PL  8    12 

MARINUS,  Lunar  Object  No.  18,  PI.  26  30 

MARIUS,  Lunar  Object  No.  390,  PI.  24, 36 32 

MARS,  Chart  of,  PL  8 11 

MARS,  Dimensions,  &c.,  PL  5  7 

MARS,  How  to  find 42 

MARS,  Index  to 42,  43 

MARS  IN  OPPOSITION  38 

MARS,  Orbit  of,  PI.  3,  4 5 

MARS,  Periodic  time  of,  PL  2; 

MARS,  Phases  of,  PL  6   8 

MARS,  SATELLITES  of,  PL  7 10 

MASKELYNE,  Lunar  Object  No.  157,  PL  23  31 

MASON,  Lunar  Object  No.  213,  PL  23    31 

MASSES  of  Stars  compared  with  Sun 68 

MAUPERTUIS,  Lunar  Object  No.  424,  PL  24 32 

MAUROLYCUS,  Lunar  Object  No.  121,  PL  26,  30, 

31    30 

MAURY,  Lunar  Object  No.  223,  PL  23    31 

McCLURE,  Lunar  Object  No.  37,  PL  26  30 

MEAN  DISTANCE  OF  A  PLANET  5 

MEDII  SINUS,  in  Moon,  P,  PL  24.  32,  33,  34 33 

MEDUSA,  PI  4 6 

MENELAUS,  Lunar  Object  No.  234,  PL  23,  31    ...  31 
MENSA,  PL  70. 

MERCATOR,  Lunar  Object  No.  339,  PL  25,  34 32 

MERCURIUS,  Lunar  Object  No.  180,  PL  23, 27,  28  31 

MERCURY,  Dimensions  of,  PL  5    7 

MERCURY,  Orbit  of,  PL  3  ...  5 

MERCURY,  Periodic  time  of,  PL  2. 

MERCURY,  when  to  be  seen,  38,  39 

MERIDIAN. — At  any  place  the  Celestial  Meridian 

is  the  great  circle  through  the  Poles  and  the 

Zenith. 


INDEX. 


91 


MERIDIAN  (CENTRAL). — Means  in  this  Atlas  the 
line  joining  the  Nortli  point  to  the  South 
point  on  the  monthly  maps. 

MEROPE,  in  Pleiades,  Ph80 62 

MERSENIUS,  Lunar  Object  No.  350,  PI.  25,  36,  37  32 
MESSALA,  Lunar  Object  No.  175,  PI.  23,  27,  28  31 
MESSIER,  Lunar  Object  No.  29,  PI.  26,  28, 29  ...  30 

METIUS,  Lunar  Object  No.  44,  PI.  26,  28,  29  30 

METON,  Lunar  Object  No.  197,  PI.  23, 30  31 

MICROMETER — An  instrument  for  the  measure- 
ment of  small  quantities. 
MiCRoscoi'iUM,  PI:  69. 

MILICHIUS,  Lunar  Object  No.  379,  PI.  24,  35 32 

MILLER,  Lunar  Object  No.  134,  PI.  26 30 

MILKY  WAY,  Photograph  around  Cluster  M.  11, 

PI.  81 62 

MIMAS,  Satellite  of  Saturn,  PI.  7 11 

MOIGNO,  Lunar  Object  No.  195,  PI.  23    31 

MONOCEROS,  PI.  59,60: 

MONOCEROTIS,  11,  a  Triple  Star,  PI.  59  74 

MONTHLY  MAPS  OF  STARS,  PI.  39-50  35 

MOON,  Elger's  drawings  of,  PI.  23-38. 
MOON,  Key  Map  of  Objects,  PI.  27-38. 

MOON,  List  of  Lunar  Objects  30-33 

MOON,  List  of  Mountain  Ran  jes 33 

MOON,  List  of  Seas  in 33 

MOON,  Mountains  near  limb  of    33 

MOON,  Orbit  of,  PL  7 10 

MOON,  Photographs  of,  PI.  20,  21,  22    25 

MOON,  Plates  of,  at  different  Phases,  PI.  27-38. 
MOON,  Sectional  Charts  of,  PI.  23-26. 

MOON,  Table  for  finding  Place  with  Age   27 

MOON'S  PHASES  AND  ECLIPSES,  PI.  14  17 

MORETUS,  Lunar  Object  No.  300,  PI.  25,  32,  33...  32 

MORTIS  LACUS,  in  Moon,  PI.  23, 23,  30,  33 33 

MOSTIXG,  Lunar  Object  No.  261,  PI. 25, 32,  33  ...  31 

MUSCA,  PI.  67,  70. 

MUTUS,  Lunar  Object  No.  53,  PI.  26,  29 30 


NADIR. — The  point  of  the  celestial  sphere  be- 
neath out  feet  to  which  a  plummet  points. 

NASIREDDIN,  Lunar  ObjectNo.287,  PI.  25 31 

NEANDER,  Lunar  Object  No.  43,  PI,  26,  28 30 

NEAP  TIDE,  Cause  of,  PI.  2   3 

NEARCHUS,  Lunar  Object  No.  50,  PI.  26 30 

NEBDLARUM   PALUS,  in  Moon,  X,  PI.  23,  31,  32  33 

NECTARIS  MARE,  in  Moon,  F,  PI.  26,  29,  30,  31  33 

NEPER,  Lunar  Object  No.  139,  PI.  23,  27 31 

NEPTUNE,  Dimensions  of,  PI.  5  7 

NEPTUNE,  Orbit  of,  PI.  4 6 


NEPTUNE,  Satellite  of,  PI.  7 11 

NEWCOMB,  Lunar  Object  No.  225,  PI.  23 31 

NEWTON,  Lunar  Object  No.  302,  PI.  25,  34  32 

NEW  MOON,  Plate  7. 

NEW  STARS  54 

NICOLAI,  Lunar  Object  No.  69,  PI.  26    30 

NICOLLET,  Lunar  Object  No.  344,  PI.  25,  33    ...     32 
NODE. — A  point  in   which  an  Orbit  intersects 
the  plane  to  which  it  is  referred. 

NONIUS,  Lunar  Object  No.  117,  PI.  26  30 

NORMA,  PI.  68. 

NORM*;,  Nova  1893,  PI.  68  54 

NUBIUM  MARE,  in  Moon,  Q,  PI.  25,  33,  34,  35, 

36,  37 33 

NUTATION. — A  small  oscillation  in  the  direction 
of  the  Earth's  axis,  due  to  the  fact  that  the 
forces  producing  precession  do  not  act  uni- 
formly. 

OBERON,  Satellite  of  Uranus,  PL  7     11 

OBLATE.  — Applied  to  a  globular  body  flattened 

at  the  poles,  like  the  Earth. 
OBLIQUITY  OF  THE  ECLIPTIC. — The  inclination  of 

the  Ecliptic  to  the  Equator. 

OBSERVATORY,  Companion  to  the  (quoted)  67 

OCCULTATION. — Applied  to  the  passage  of  the 

Moon  over  a  star,  or  of  Jupiter  over  one  of 

his  satellites. 
OCEANUS  PROCELLARUM,  in  Moon,  S,  PI.  24,  35, 

36,37,38  33 

OCTANS,  PI.  70. 

(ENOPIDES,  Lunar  Object  No,  435,  PI.  24 32 

OERSTED,  Lunar  Object  No.  183,  PI.  23 31 

OKEN,  Lunar  Object  No  20.  PI.  26 30 

OLBERS,  Lunar  Object  No.  393,  PL  24,  38 32 

OPHIUCHI,  Nova  1848,  PI.  62  , 54 

OPHIUCHUS,  PI.  62,  68,  71,  72. 

OPPOLZER,  Work  on  Eclipses   18 

OPPOSITION — a  Planet  is  in  opposition  to  the 

Sun  when  its  longitude  differs  from  that  of 

the  Sun  by  180°.     Symbol  lor  opposition...      5 
ORBIT.  — The  track  pursued  by  a  planet  round 

the  Sun,  or  by  a  Satellite  round  its  primary 

planet. 

ORCUS  (Canal),  on  Mars,  PI.  8 12 

ORIANI,  Lunar  Object  No   163,  PL  23  31 

ORION,  PI.  59,  71,  72. 

ORION,  Great  Nebula  in,  PI.  73  59 

ORIONIS,  a,  or  BETELGEUX,  PI.  59,  Variable... 54,  74 
ORIONIS,  8,  Multiple  Star 73 


92 


INDEX. 


ORIONIS,  Z,  Triple  Star,  PI.  59    73 

ORIONIS,  ff,  MultipleStar 73 

ORONTIUS,  Lunar  Object  No.  288,  PI.  25  31 


PALITZSCH,  Lunar  Object  No.  9,  PL  26 30 

PALLAS,  Lunar  Object  No.  375,  PL  24,  32 32 

PALLAS,  minor  Planet ,  diameter  of 8 

PALUS  NEBULARUM,  in  Moon,  X,  PL  23,  31,  32  33 

PALUS  PUTREDINIS,  in  Moon,  Z,  PL  23,  32 33 

PALUS  SOMNII,  in  Moon,  V,  PL  23,  28,  29    33 

PARALLAX. — The  difference  in  direction  between 
the  positions  of  a  heavenly  body  as  seen 

from  two  different  points,  PI.  1  2 

PARALLELS. — Circles  parallel  to  the  Equator, 
having  one  of  the  Poles  as  centre. 

PARROT,  Lunar  Object  No.  108,  PL  26 30 

PARRY,  Lunar  Object  No.  271,  PL  25,  33 31 

PATHS  OF  SPOTS  ACROSS  THE  SUN'S  Disc,  PL  13  16 

PAVO,  PL  68,  69,  70,  72. 

PAVONIS,  K,  regularly  Variable  Star,  PI.  69,  70  53 

PEGASUS,  PL  52,  57,  58,  63,  71,  72. 

PEIRCE,  Lunar  Object  No.  153,  PL  23,  27, 28,  29  31 

PENTLAND,  Lunar  Object  No.  131,  PL  26 30 

PERCY,  MTS.,  in  Moon,  n,  PL  25    33 

PERIHELION  OF  A  PLANETARY  ORBIT 5 

PERIODIC  TIME  IN  A  PLANETARY  ORBIT 2 

PERRINE'S  COMET,  1902,  III.,  PL  19 23 

PERSEI,  /3,  or  ALGOL,  regularly  Variable  Star, 

PL  53  51,  72 

PERSEI,  0,  a  Triple  Star,  PL  53  72 

PERSEI,  p,  Variable  Star,  PL  53 54 

PERSEI,  Nova  1887,  PL  53 54 

PERSEI,  Nova  1901,  PL  53 54 

PERSEI,  Nova  1901,  Photographs  of  Star  and 

surrounding  Nebula,  PL  82 63 

PF,RSEIDS,  Orbit  of,  PL  3 5 

PERSEIDS,  Meteor  Shower    3,55 

PERSEUS,  PL  52,  53,  71. 

PERSEUS,  Great  Clusters  in,  PL  52,  53 71 

PERTURBATION. — A  disturbance  in  the  orbit  of  a 
heavenly  body  caused  by  some  other  attrac- 
tion besides  that  which  chiefly  controls  the 
motion. 

PETAVIUS,  Lunar  Object  No.  7,  PL  26,  27,  28,  29  30 

PETERS,  Lunar  Object  No.  196,  PL  23  31 

PHASES  OF  THE  MOON,  PL  14  17 

PHASES  OF  THE  PLANETS,  PL  6    8 

PHILLIPS,  Lunar  Object  No.  13,  PI.  26 30 


PHILOLAUS,  Lunar  Object  No.  420,  PL  24,  34, 

35,  36 '....'  32 

PHOBOS,  Satellite  of  Mars,  PL  7 10 

PHOCYLIDES,  Lunar  Object  No.  321,  PL  25,  36, 

37,  38 32 

PHOEBE,  Satellite  of  Saturn n 

PHCENIX,  PL  64,  65,  72. 

PIAZZI,  Lunar  Object  No.  329,  PI.  25,  37,  38  ...  32 

PICARD,  Lunar  Object  No.  152,  PL  23,  27,  28,  29  31 

PICCOLOMINI,  Lunar  Object  No.  63,  PL  26, 29,  30  30 
PICKERING,  W.  H.,  Photograph  of  Solar  Corona, 

PI  16  19 

Pico,  Lunar  Mountain,  q,  PL  24,  33,  34.  35 33 

PICTET,  Lunar  Object  No.  289,  PL  25    32 

PICTOR,  PL  65,  66. 

PINGRE,  Lunar  Object  No.  319,  PL  25 32 

PISCES,  PL  52,  58,  63,  71,  72   3 

PISCIS  AUSTRALIS,  PL  64,  69,  72. 

PISCIUM,  35,  Double  Star,  PL  58,  63 70 

PISCIUM,  a,  Double  Star,  PI.  58  71 

PITATUS,  Lunar  Object  No.  280,  PL  25,  33  31 

PITISCUS,  Lunar  Object  No.  52,  PL  26,  29    30 

PITON,  Lunar  Mountains,  r,  PL  24,  34,  35  33 

PLANA,  Lunar  Object  No.  214,  PL  23,  29 31 

PLANET,  Naming  an  unknown 48 

PLANETS,  Inner,  PL  3    4 

PLANETS,  Comparative  Sizes  of,  PL  5    7 

PLANET,  Mean  Distance  of  a 5 

PLANETARY  PHENOMENA  38 

PLANETS,  Symbols  for  the 5 

PLATO,  Lunar  Object  No.  413,  PL  24,  32,  33, 

34,  35  29,  32 

PLAYFAIR,  Lunar  Object  No.  100,  PL  26,  31   ...  30 
PLEIADES,  Photographs  of  Stars  and  Nebulae, 

PL  79-80    62 

PLEIADES,  PL  53,  58,  71,  72,  79,  80   72 

PLINIUS,  Lunar  Object  No.  227,  PL  23,  30,  31...  31 

PLUTARCH,  Lunar  Object  No.  164,  PL  23 31 

POISSON,  Lunar  Object  No.  96,  PL  26   30 

POLAR  Axis,  PL  1  1 

POLE  STAR,  as  a  Double  Star,  PL  51  ....*. 70 

POLYBIUS,  Lunar  Object  No.  92,  PL  26 30 

PONS,  Lunar  Object  No.  93,  PL  26,  30  30 

PONTANUS,  Lunar  Object  No.  94,  PL  26   30 

PONTECOULANT,  Lunar  Object  No.  22,  PL  26,  27, 

28    30 

POSIDONIUS,  Lunar  Object  No.  218,  PL  23,  29, 

30    31 

POSITION  ANGLE  OF  THE  SUN'S  A  xis  16 

PR^SEPE  CANCRI— a  Cluster,  PL  54,  60    75 


INDEX. 


93 


PKKCESSION  IN  DECLINATION,  Table  of 57 

PRECESSION  in  R.  A.,  Table  of 57 

PRECESSION  OF  THE  EQUINOXES. — An  alteration 
in  the  position  of  the  Equinoxes,  due  to  a 
continuous  revolution  of  the  pole  of  the 
Equator  round  the  pole  of  the  Ecliptic,  in 

about  '26,000  years 56 

PRIME  VERTICAL,  PI.  1. 

PROCELLARUM  OCEANUS,  in  Moon,  S,  PI.  24,  35, 

36,37,38  33 

PROCLUS,  Lunar  Object  No.  156,  PI.  23,  28,  29    31 

PROMINENCES  on  the  Sun,  PI.  12 15 

PROMONTORY,  LAPLACE,  in  Moon,  u,  PL  24,  34, 

35,  36,  37  33 

PTOLEM,>:US,  Lunar  Object  No.  264,  PL  25,  32,  33    31 
PUPPIS,  PI.  59,  60,  66. 

PUPPIS  L2?  regularly  Variable  Star,  PL  66   53 

PUPPIS  V,  Variable  Star,  PL  66 53 

PURBACH,  Lunar  Object  No.  277,  PI.  25,  32 31 

PUTREDINIS  PALUS,  Z,  in  Moon,  PL  23,  32  33 

PYRENEES,  in  Moon,  g.  PL  26,  28,  29,  30 33 

PYHIPHLEGETHON,  on  Mars.  PL  8  12 

PYTHAGORAS,  Lunar  Object  No.  432,  PL  24,  37, 

38    32 

PYTHEAS,  Lunar  Object  No.  403,  PL  24,  33,  34    32 


QUADRATURE. — The  position  of  a  heavenly  body 

when  90°  from  the  Sun....  5 


RABBI  LKVI,  Lunar  Object  No.  66,  PL  26,  30...     30 
RADIANT  POINT. — The  point  on  the  heavens  from 

which  the  Shooting  Stars,  in  a  shower  of 

such  bodies,  appear  to  diverge. 
RAMBAUT,  Dr.  Arthur  A.     See  Preface. 
RAMSDEN,  Lunar  Object  No.  355,  PL  25,  34,  35    32 

REAUMUR,  Lunar  Object  No.  115,  PL  26 30 

RED  SPOT  OD»JUPITER,  PI. 9 0 12 

REFRACTION.— The  bending  of  a  ray  of  light  on 

passing  from  one  medium  into  another 2 

REFRACTIONS,  Table  of 2 

REGIOMONTANUS,  Lunar  Object  No.  278,  PL  25, 

32  - 31 

REGULARLY  VARIABLE  STARS 51 

REICHENBACH,  Lunar  Object  No.  41,  PL  26,  28    30 

REINER,  Lunar  Object  No.  389,  PL  24,  38    32 

REINHOLD,  Lunar  Object  No.  377,  PL  24,  33,  34, 

35   32 

REPSOLD,  Lunar  Object  No.  439,  PL  24,  38 33 

RETICULUM,  PL  65,  70. 


PAGK. 

RETROGRADE — applied  to  the  motion  of  a  planet 
or  satellite  when  it  is  in  the  direction  oppo- 
site to  the  general  direction  of  motion. 

RH.KTICUS,  Lunar  Object  No.  114,  PL  26,  31  ...  30 

RHEA,  Satellite  of  Saturn,  PI.  7 '..  11 

RHEITA,  Lunar  Object  No.  42,  PI.  26    30 

RICCIOLI,  Lunar  Object  No.  365,  PL  25,  38 32 

RICCIUS,  Lunar  Object  No.  65,  PL  26,  30 30 

RIGHT  ASCENSION  1 

RIPH/EAN  MOUNTAINS,  in  Moon,  i,  PL  25,  35  ...  33 
RITCHEY,  G.  W.,  Photograph  of  prominences, 

PL  12 15 

RITCHEY,  G.   W.,  Photographs  of  the  Moon, 

PI.  20,  21,  22   25 

RITCHEY,  G.  W.,  Photograph  of  Orion  Nebula, 

PL  73 59 

RITCHEY,   G.   W.,  Photograph  of  Andromeda 

Nebula,  PL  74 '59 

RITCHEY,  G.  W.,  Nova  Persei  Nebula,,  PL  82  ...  68 

RITTER,  Lunar  Object  No.  246,  PL  23,  30 31 

ROBERTS,    Isaac,    Photograph    of    Nebulae    in 

Pleiades,  PL  79   62 

ROBINSON,  Lunar  Object  No.  436.  PL  24  32 

ROCCA,  Lunar  Object  No.  362,  PL  25,  37,  38  ...  32 

ROMER,  Lunar  Object  No.  221,  PL  23  31 

ROOK  MOUNTAINS,  Lunar  Object    33 

RORIS  SINUS,  in  Moon,  W,  PL  24,  37,  38 33 

ROSENBERGER,  Lunar  Object  No.  49,  PL  26, 28, 

29    30 

Ross,  Lunar  Object  No.  228,  PL  23,  30 31 

ROSSE,  Lunar  Object  No.  61,  PL  26  30 

ROST,  Lunar  Object  No.  3150P1.  25,  34,  35 32 


SABINE,  Lunar  Object  No.  247,  PL  23,  30    31 

SACROBOSCO,  Lunar  Object  No.  90,  PL  26,  30 ...  30 
SAGITTA,  PL  62,  63. 

SAGITTJE,  S,  Variable  Star,  PL  6^2  53 

SAGITTARII,  X  3,  regularly  variable  Star,  PL  68  53 
SAGITTARII,  Wyi    regularly  Variable  Star,  PL 

68,69 53 

SAGITTARII,  Nova  1898,  PL  62 54 

SAGITTARII,  Y,  Variable  Star  PL  62 53 

SAGITTARIUS,  PL  62,  68,  69,  72-  9 

SAGITTARIUS,  Nebula  M.  17  in,  PL  62   79 

SAGITTARIUS,  Cluster  M.  8  in,  PL  62,  69  79 

SANTBECH,  Lunar  Object  No.  36,  PL  26,  28,  29  30 

SAROS,  The  Eclipse  Cycle 18 

SASSERIDES,  Lunar  Object  No.  284,  PL  25,  33...  31 

SATELLITES,  Systems  of,  PL  7 ,..  10 

SATURN,  Description  of  Orbit  of,  PL  4  g 


94 


INDEX. 


SATURN,  Description  of,  PI.  10    13 

SATURN,  Dimensions  of  Rings,  PI.  5  7 

SATURN,  Drawing  of,  PI.  10 13 

SATURN  IN  OPPOSITION,  to  1950  38 

SATURN,  Index  to 46,  47 

SATURN,  Orbit  of,  PI.  4. 

SATURN,  Phases  of  Rings,  PI.  6 9,  38 

SATURN,  Satellites  of,  PI.  7  11 

SAUSSURE,  Lunar  Object  No.  290,  PI.  25,  32,  33  25,  32 
SCHEINER,  Lunar  Object  No.  313,  PI.  25,  34,  35  32 
SCHIAPARELLI,  Lunar  Object  No.  450,  PI.  24  ...  33 
SCHICKARD,  Lunar  Object  No.  323,  PI.  25,  36, 

37,38     32 

SCHILLER,  Lunar  Object  No.  317,  PI.  25,  35,  36  32 
SCHOMBERGER,  Lunar  Object  No.  27,  PI.  26,  29  30 
SCHROTER,  Lunar  Object  No.  374.  PI.  24,  32  ...  32 

SCHUBERT,  Lunar  Object  No.  135,  PI.  23  30 

SCHUMACHER,  Lunar  Object  No.  178,  PL  23 31 

SCHUSTER,   A.,   Photograph  of   Solar  Corona, 

PI.  16 19 

SCHWABE,  Lunar  Object  No.  149,  PI.  23    31 

SCORESBY,  Lunar  Object  No.  202,  PI.  23,  30 31 

SCORPII,  T.Nova  (1860),  PL  61,  62.68    54 

SCORPII,  a,  or  ANTARES,  PI.  68  78 

SCORPII,  IJL,  Spectroscopic  Binary   ^..     79 

SCORPIO,  PI.  61,  62,  68,  72. 
SCULPTOR,  PL  58, 64. 

SCUTI,  R.,  Variable  Star,  PL  62 54 

SEASONS,  Cause  of,  PL  2    1 

SECCHI,  Lunar  Object  No.  155,  PL  23, 28    31 

SEGNER,  Lunar  Object  No.  311,  PL  25,  35 32 

SELEUCUS,  Lunar  Object  No.  397,  PL  24,  37,  38    32 

SENECA,  Lunar  Object  No.  165,  PL  23    31 

SERENITATIS  MARE,- in  Moon,  J,  PI.  23,  30,  31, 

32   33 

SERPENS,  PL  56, 61,  62,  71,  72. 

SERPENTARII,  Nova  1604,  PI.  62  54 

SEXTANS,  PL  60.     * 

SHARP,  Lunar  Object  No.  426,  PL  24,  35,  36 32 

SHORT,  Lunar  Object  No.  301,  PL  25 32 

SHORT  PERIOD  VARIABLES 52  53 

SHUCKBURGH,  Lunar  Object  No.  184,  PI.  23    .  .    31 
SIDEREAL  DAY 35 

SILBERSCHLAG,  Lunar  Object  No.  252,  PL  23  31 

SIMPELIUS,  Lunar  Object  No.  133,  PI.  26  30 

SINUS  ^ESTUUM,  in  Moon,  N,  PL  24,  32,  33,  34...  33 

SINUS  IRIDUM,  in  Moon,  R,  PI.  24,  34, 35,  37,  38  33 
SINUS  IRIDUM  HIGHLANDS,  in  Moon,  e,  PL  24,  35, 

36- 37 '     '  33 

SINUS  MEDII,  in  Moon,  P,  PL  24, 32,  33  33 


Six  us  RORIS,  in  Moon,  W,  PL  24,  37,  33  .    33 

SIRENIUS  LACUS,  on  Mars,  PI.  8  ...  12 

SIRIUS,  Binary  Star  .................  7-4 

SIUSALIS,  Lunar  Object  No.  358,  PL  25  .32 

SMYTH,  PIAZZI.  Lunar  Object  No.  412,  PI.  24  '     32 

SMYTHII  MARE,  in  Moon,  Y,  PL  23.  27*..  33 

SMYTH'S  CELESTIAL  CYCLE   .........  ...  '    QJ 

SNELLIUS,  Lunar  Object  No.  16,  PI.  26,  28  '    30 

SOLAR  CORONA,  Description  of,  PI.  16....  jg 

SOLAR  PHENOMENA,  Corona   and  Prominences 


;.        ..........................................  15,18 

SOLIS  LACUS,  on  Mars,  PL  8    .........  12 

SOLSTICES.—  The  points  of  the  Ecliptic  attained 
by  the  Sun  at  Midsummer  and  Midwinter, 


PL  1 


1 


SOMMERING,  Lunar  Object  No.  373,  Pi.  24,  32  32 

SOMNH  PALUS,  in  Moon,  V,  PI.  23, 28  ...  33 

SOMNIORUM  LACUS,  in  Moon,  G,  PL  23..  29,  30,  31  33 

SOSIGENES,  Lunar  Object  No.  230,  PL  23  25 

SOUTH,  Lunar  Object  No.  433,  PI.  24....             '  32 

SPECTROSCOPY gg 

SPECTROSCOPIC  BINARIES  gg 

SPHERE,  Circles  of  the,  PL  1. 

SPHERE  OF  THE  HEAVENS  j 

SPRING  TIDE,  PL  2 3 

STADIUS,  Lunar  Object  No.  381,  PI.  24, 33  32 
STAR  MAPS,  PL  51-72. 

STAR  MAPS,  Description  of 49-58 

STAR  MAPS,  correction  Jor  Precession    58 

STARS,  Variable,  Lists  of ....51-54 

STEINHEIL,  Lunar  Object  No.  47,  PL  26 30 

STEVINUS,  Lunar  Object  No.  15,  PL  26,  28         .  30 

STIBORIUS,  Lunar  Object  No.  64,  PL  26,  29,  30...  30 

STOFLER,  Lunar  Object  No.  119,  PL  26,  31  30 

STRABO,  Lunar  Object  No.  190,  PL  23 31 

STRAIGHT  RANGE,  Lunar  Mountains,  m,  PL  24  33 
STRAIGHT  WALL,  Lunar  Object  No.  275.  PL  25, 

33  31 

STREET,  Lunar  Object  No.  295,  PL  25 32 

STRUVE,  Lunar*0bject  No.  179    *..          .  31 

STRUVE,  Otto,  Lunar  Object  No.  446,  PL  24,  38  33 
SULPICIUS  GALLUS,  Lunar  Object  No.  235,  PL  23, 

31  31 

SUMMER  SOLSTICE,  PL  2    i 

SUN,  Corona  of,  PL  16    y .'....  18 

SUN,  Eclipse  of,  PL  14   .'. 17 

SUN,  Node  of  Solar  Equator,  PL  3. 

SUN,  Paths  of  Spots  across  face,  PL  13  16 

SUN,  Paths  of  Total  Eclipses,  PI.  15  18 

SUN,  Photograph  of  Sun  Spot,  PL  11 15 

SUN,  Prominences  surrounding,  PL  12    .  ,15 


INDEX. 


95 


SUN  SPOT,  Photograph  of,  PI.  11 15 

SYKTIS  MINOR,  on  Mars,  PI.  8     12 

SVRTIS  MAJOR,  on  Mars,  PI.  8    12 


TACITUS,  Lunar  Object  No.  82,  PI.  26,  30 30 

TANNERUS,  Lunar  Object  No.  145,  PI.  26 31 

TAQUET,  Lunar  Object  No.  233,  PI.  23,  31   31 

TARUNTIUS,  Lunar  Object  No.  154,  PI.  23,  28, 

29    31 

TAURI,  a,  or  ALDEBARAN,  PI.  59 73 

TAURI,  X,  Algol  Variable,  PI.  58,  59 52 

TAURUS,  Lunar;Moimtains,  k,  PL  29,  30,  31  ...  33 
TAURUS.  PI.  53,  58,  59,  71,  72. 

TAURUS,  Nebula,  M.  1  in,  PI.  53,  59 73 

TAYLOR,  Lunar  Object  No.  76,  PI.  26   30 

TELESCOPIUM,  PL  68,  69. 

TEMPORARY  STABS 54 

TENERIFFE  RANGE,  Lunar  Mountains,  I,  PI.  24  33 

TERMINATOR  OF  MOON 28 

TETHYS,  Satellite  of  Saturn,  PI.  7 11 

THALES,  Lunar  Object  No.  191,  PI.  23 31 

THE^ETETUS,  Lunar  Object  No.  257,  23,  31,  32...  31 

THEBIT,  Lunar  Object  No.  274,  PI.  25,  32,  33...  31 

THEON,  JUN.,  Lunar  Object  No.  75,  PI.  26 30 

THEON,  SEN.,  Lunar  Object  No.  74,  PL  26  30 

THEOPHILUS,  Lunar  Object  No.  79,  PL  26,  30...  30 

TIDES,  PL  2 3 

Tisucus,  Lunar  Object  No.  414,  PL  24,  32  32 

TIM*:,  Standard,  PL  83 82 

TIMOCHARIS,  Lunar  Object  No.  401,  PL  24,  33 

34,  35 32 

TIMOLEON,  Lunar  Object  No.  147,  PL  23 31 

TITAN,  Satellite  of  Saturn,  PL  7     11 

TITANIA,  Satellite  of  Uranus,  PI.  7    11 

TOBIAS  MAYER,  Lunar  Object  No.  384,  PL  24, 

34,  35 32 

TORRICELLI,  Lunar  Object  No.  56,  PL  26 30 

TOTAL  ECLIPSE  OF  SUN,  PL  16 18 

TOUCANA,  PL  70. 

TOUCANI,  47,  Globular  Cluster    70 

TRALLES,  Lunar  Object  No.  168,  PL  23,  28 31 

TRANQUILLITATIS  MARE,  in  Moon.E,  PL  23,29, 

30,  31 33 

'TRANSIT. — The  passage  of  a  celestial  body  across 

a  fixed  line,  of  a  planet  across  the  Sun,  or 

of  one  of  his  satellites  across  Jupiter. 
TRIANGULA,  PL  52,  53,  71. 

TRIANGULI  i,  a  Double  Star,  Pi.  52,  53 71 

TPIANGULUM,  PL  68,  70,  72. 


TRIESNECKER,  Lunar  Object  No.  242  PI  23  31 

'** 31 

TRIVIUM  CHARONTIS.  on  Mars,  PI.  8  ...  ]2 

TURNER,  H.  H. ,  Discoverer  of  Nova  Geminorum  54 
TYCHO,  Lunar  Object  No.  291,  PI.  21,  25,  33, 

34     25.32 


UKERT,  Lunar  Object  No.  241,  PI.  23   31 

ULUGH  BEIGH,  Lunar  Object  No.  433,  PL  24  ...  33 

UMBRIEL,  Satellite  of  Uranus,  PI.  7   n 

URANOMETRIE  GENERALE  of  HOUZEAU f>0 

URS.E  MAJORIS,  £,  a  Double  Star,  PI.  55 76 

URS.E  MAJORIS,  £,  a  Double  Star,  PL  55 77 

URSA  MAJOR,  PL  51,  54,  55,  56,  71. 

URSA  MAJOR,  Planetary  Nebula,  M.  97,  PL  55  76 

URSA  MINOR,  PL  51,  56,  71. 

URS.E  MINORIS,  a,  as  a  Double  Star,  PL  51 70 

URANUS,  Dimensions  of,  PL  5 7 

URANUS,  Orbit  of,  PI.  4 6 

URANUS,  Satellites  of,  PL  7 11 


VAPORUM  MARE,  in  Moon,  M,  PI.  23,  31  .........     33 

VARIABLE  STARS  .......................................  51-55 

VASCO  DE  GAMA,  Lunar  Object  No.  396,  PL  24 
VEGA,  Lunar  Object  No.  21,  PL  26    ............... 

VEGA,  or  a  LYR.W,  PL  57  .............................. 


32 
30 

79 
63 


VELOCITY  of  Star  at  right  angles  to  line  of  Sight 

VELA,  PL  66,  67. 

VELORUM  N.,  Variable  Star,  PL  66   ...............  53 

VENDELINUS,  Lunar  Object  No.  3,  PL  26,  27, 

28,  29  ...................................................  30 

VENUS,  as  an  Evening  Star  ...........................  38 

VENUS,  as  a  Morning  Star    ...........................  38 

VENUS,    Dimensions   of,   and  position   of  Axis, 

PL  5  ...    ..............................................  7 

VENUS,  how  to  find    ....................................  40 

VENUS,  Index  to   .......................................  39-41 

VENUS,  Orbit  of,  PL  3. 

VENUS,  Periodic  Time  of,  PL  3. 

VENUS,  Phases  of,  PL  6  .................................  8,  9 

VERTICAL,  PRIME,  PL  1. 

VESTA,  diameter  of  .......................................  8 

VIETA,  >  unar  Object  No.  332,  PL  25,  36,  37    ...  32 

VIRGINIS,  a,  (Spica),  Spectro.scopic  Binary  ......  77 

VIRGINIS7,  a  Double  fctar.  PL  61  ..................  77 

VIRGO,  PL  60,  61,71,  72   ..............................  3 

VITELLO,  Lunar  Object  No.  335,  PL  25,  35  ......  32 

VITRUVIUS,  Lunar  Object  No.  159,  PI.  23,  29  31 

VLACQ,  Lunar  Object  No.  48,  PI.  26,  29    ......  30 


96 


INDEX. 


TAKE. 

VOLANS,  PI.  66,  70. 
VULPECDLA,  PI.  57,  62,  6:j. 

VCLPECUL.S;,  Nova,  1670,  PI.  57 54 

VULPECULA,  Dumb-bell   Nebula  in,  PI.  57,62, 

77    61 

"VULPECUUE  T.,  regularly  Variable  Star,  PI.  57     53 


WALTER,  Lunar  Object  No.  116,  PL  26,  32 30 

WAKGENTIN,  Lunar  Object  No.  322,  PI.  25,37,38  32 

WEBB,  Lunar  Object  No.  28,  PI.  26  30 

WEIGEL,  Lunar  Object  No.  314,  PI.  25 32 

WEKNER,  Lunar  Object  No.  98,  PI.  26,  31,  32  30 

WHEWELL,  Lunar  Object  No.  255,  PI.  23 ol 

WICHMANN,  Lunar  Object  No.  369,  PI.  25,  35  32 

WILHELM  I.,  Lunar  Object  No.  293,  PI.  25,  33,  34  32 
WILHELM  HUMBOLDT,  Lunar  Object  No.  12, 

PI.  26,27 30 

WILLIAMS,  A.  STANLEY,  Photographs  of  Nova 

Persei,  PI.  82  63 

WILSON,  W.  E.,  Photographs  of  Cluster  in 

Hercules  and  Nebula  in  Cygnu?,  PI.  75    ...  60 


WILSON,  Lunar  Object  No.  307,  PL  25,  35 32' 

WOLLASTON,  Lunar  Object  No.  449,  PL  24,  36...  33- 

WROTTESLEY,  Lunar  Object  No.  8,  PL  26    30 

WUR/ELBAUER,  Lunar  Object  No.  283,  PL  25,  33  31 


XENOPHANES,  Lunar  Object  No.  438,  PL  24,  38    32' 


ZACH,  Lunar  Object  No.  129,  PL  26.  31,  32 30- 

ZAGUT,  Lunar  Object  No.  67,  PL  26,  30    30' 

ZENITH.  —  The  point  of  the  celestial  sphere 
directly  overhead  to  which  a  plumb  line 
points. 

ZENO,  Lunar  Object  No.  148,  PI,  23 31 

ZODIAC. — A  belt  on  the  heavens  within  which 
the  larger  planets  chiefly  remain.  It  is 
practically  what  is  marked  on  the  Monthly 
Maps  as  the  "Track  of  the  Planets." 

ZODIAC,  Signs  of  the 4 

ZUCHIUS,  Lunar  Object  No.  310,  PL  25 32 

ZUPUS,  Lunar  Object  No.  360,  PI.  25 32. 


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